August is water month. Whether you're at the beach, by a river, or just armed with a garden hose on a warm afternoon, water is everywhere—and it is, without exaggeration, the most extraordinary substance in existence. Water is the only common substance that exists naturally in all three states (solid, liquid, gas) at temperatures humans regularly experience. It expands when it freezes—uniquely, bizarrely, life-savingly—meaning ice floats and lakes freeze from the top down rather than the bottom up, preserving the liquid water beneath that aquatic life depends on. It has an absurdly high surface tension that allows insects to walk on it and water striders to sprint across ponds. It dissolves more substances than any other liquid, earning its title as the "universal solvent." And it absorbs and releases heat with extraordinary efficiency, moderating Earth's climate and making coastal regions more temperate than inland areas at the same latitude.
This month's experiments celebrate water's extraordinary properties. You'll investigate surface tension in ways that will astonish anyone watching, explore water's density behaviour in a way that creates vivid rainbow columns in a glass, and investigate the strange physics of non-Newtonian fluids—mixtures that behave as liquid one moment and solid the next. These are experiments that work on warm August afternoons, can involve as many people as you have glasses for, and guarantee the kind of genuine surprise that reminds everyone—children and adults alike—that science is essentially the business of noticing how remarkable the world actually is.
Experiment 1: The Magic of Surface Tension
What You'll Learn
Explore water's extraordinary surface tension—the skin-like property that allows insects to walk on water, gives water droplets their spherical shape, and determines how water moves through plant stems. This experiment reveals the molecular forces that make water's surface behave like an elastic membrane.
Equipment Needed
- A large bowl or baking tray (the wider the better)
- Water
- Washing-up liquid
- A needle or straightened paperclip
- Black pepper or talcum powder
- Small pieces of card cut into boat shapes (5cm × 3cm, with a triangular notch cut from one end)
- Optional: a coin (to see how many drops fit on its surface)
- Optional: a loop of string tied loosely (for the soap film demonstration)
- Tissue paper (one small square)
Safety note: No significant hazards. Washing-up liquid is a mild irritant—avoid contact with eyes.
Method
1. Float a needle (5 minutes)
Fill your bowl with cold water and allow the surface to settle completely. Cut a small square of tissue paper, about 3cm × 3cm. Place the tissue on the water surface. Lay your needle or straightened paperclip gently on top of the tissue.
Now, use a pencil or another needle to gently push the tissue paper down at its edges, submerging it. Done carefully, the tissue sinks whilst the needle remains floating on the surface—balanced on the invisible elastic "skin" of surface tension. The needle, denser than water, floats because the surface tension distributes its weight across an area rather than allowing it to pierce through.
Look at the floating needle from the side if possible: you'll see the water surface visibly dipping beneath it, like a stretched elastic sheet. The needle sits in a tiny hammock of surface tension.
2. The pepper demonstration (5 minutes)
Sprinkle black pepper or talcum powder generously over the surface of the water in your bowl. It should form a layer across the entire surface.
Now dip one fingertip into the water at the centre of the bowl. Nothing dramatic—the pepper just swirls slightly. Remove your finger.
Dip your fingertip into a small amount of washing-up liquid, coating just the tip. Touch this soapy finger to the centre of the water surface. Watch what happens.
The pepper explodes outward to the edges of the bowl—instantly, dramatically, as if something has detonated at the centre. This is one of science's most reliable and visually striking demonstrations.
3. The soap boat (5 minutes)
Cut small boat shapes from card—a pointed front, a rectangular body, and a triangular notch cut from the stern (back). Make several identical boats.
Place one boat on the still water surface. It floats. Now put one drop of washing-up liquid into the notch at the stern. The boat shoots forward, propelled by nothing visible.
Try it again with fresh water. Does the boat still move as dramatically on its second trip? What about a third?
4. The coin challenge (5 minutes)
Dry your coin thoroughly. Hold a dropper or straw over it and drip water onto the coin, one drop at a time. How many drops can fit on the coin's surface before overflowing? Count carefully.
The answer typically astonishes people—a 2p coin can hold 20-30 drops before the surface tension fails and water spills over. Watch from the side as drops are added: the water surface bulges upward dramatically, forming a dome well above the coin's edge before finally collapsing.
Expected Results
Floating needle: With care, the needle floats reliably. The tissue paper trick makes it easier—the tissue sinks, leaving the needle floating. A visually striking result that appears to defy gravity.
Pepper demonstration: The explosion of pepper to the edges is instantaneous and dramatic—one of the most reliably impressive effects in kitchen science. It never fails to produce a reaction from anyone watching.
Soap boat: The boat moves clearly and quickly on first attempt. Subsequent attempts in the same water are less dramatic, and eventually the boat stops moving—the reason is important (see Science Explained).
Coin drops: Most people estimate 3-5 drops; the actual number (20-30) reliably surprises. The dome of water visible above the coin's edge demonstrates surface tension spectacularly.
Troubleshooting: If the needle sinks, ensure the surface is completely still before attempting. Use colder water—surface tension is slightly higher in cold water. Handle the needle with a tissue rather than fingers, as skin oils can break surface tension. If the pepper trick doesn't work dramatically, use more pepper and ensure your soapy finger has been dipped in concentrated washing-up liquid.
The Science Explained
Surface tension arises from the cohesive forces between water molecules. Water molecules are polar—one end is slightly positively charged, the other slightly negatively charged. This polarity means neighbouring water molecules attract each other through hydrogen bonds—weak but numerous electrical attractions between the partially charged ends of different molecules.
Molecules within the bulk of the water are surrounded by other molecules in all directions, so the forces balance—no net force in any direction. But molecules at the surface have neighbours below and to the sides, but not above. They experience a net inward pull. This inward pull creates a tension across the surface—water molecules at the surface are being pulled inward and sideways, creating a surface that behaves like a stretched elastic membrane.
Water has higher surface tension than almost any other common liquid—72 millinewtons per metre at room temperature. This is why water beads on waxed surfaces, why raindrops are spherical (surface tension minimises surface area, and a sphere has the minimum surface area for a given volume), and why water rises in thin tubes (capillary action) against gravity.
The pepper explosion demonstrates how soap destroys surface tension. Washing-up liquid contains surfactants (surface-active agents)—molecules with a water-loving head and water-hating tail. Surfactants position themselves at the water surface, with tails pointing outward and heads into the water, disrupting the hydrogen bonding network and drastically reducing surface tension.
When soap touches the centre of the water surface, it lowers surface tension there immediately. But the surface around the edges still has full surface tension. The high-tension edges pull the surface outward, dragging everything on the surface—including the pepper—with them. The pepper rushes to the edges in the time it takes surface tension to equilibrate. It's not repulsion; it's the high-tension edges pulling the surface outward.
The soap boat works by the same principle. Soap released from the notch lowers surface tension behind the boat. Higher surface tension in front pulls the water surface—and the boat with it—forward. This only works once per bowl because soap quickly distributes through all the water, equalising surface tension throughout, after which there's no tension gradient to drive the boat.
Capillary action—water rising in plant stems and thin tubes—also depends on surface tension. Water's cohesion (attraction between water molecules) and adhesion (attraction between water and the tube walls) combine to pull water upward. This is the mechanism by which water moves from roots to leaves in trees—a striking application of the same molecular forces demonstrated in your bowl.
Real-World Applications
Surface tension is exploited by an extraordinary range of organisms. Water striders (pond skaters) have hairy, water-repellent legs that spread their weight over a large surface area, allowing them to walk on water without breaking the surface. Diving bell spiders carry air bubbles underwater, maintained by surface tension. Some beetles collect drinking water from fog by using surfaces that alternate between water-attracting and water-repelling sections, causing droplets to form and roll toward their mouths.
Surface tension matters industrially in inkjet printing (where surface tension controls droplet formation), in the manufacture of lenses (liquid glass uses surface tension to form precise curves), and in the extraction of oil from rock formations (where surfactants similar to washing-up liquid are injected to reduce surface tension between oil and water, allowing oil to flow more freely).
Medical applications include lung surfactant—a mixture of molecules that reduces surface tension in the lung's air sacs, preventing them from collapsing when we exhale. Premature babies sometimes lack sufficient surfactant, requiring replacement therapy. Understanding surface tension at the molecular level has informed the development of these treatments.
Taking It Further
Variation 1: Test whether temperature affects surface tension. Use hot water, room temperature water, and cold (ice) water. Surface tension decreases with temperature—cold water should hold more drops on a coin, float needles more reliably, and show more dramatic pepper response.
Variation 2: Test different liquids. Does alcohol (vodka) show the same effects as water? Alcohol has much lower surface tension—the needle won't float and fewer drops will fit on a coin. This demonstrates why alcohol-water mixtures (wines, spirits) behave differently from pure water.
Variation 3: Create a soap film on a loop of string. Carefully pop part of the film with a dry finger—the remaining film contracts, pulling the string into a circle. This demonstrates that surface tension minimises area.
Experiment 2: Rainbow in a Glass — Water Density Columns
What You'll Learn
Create a stunning rainbow of layered coloured liquids in a glass by exploiting differences in liquid density, demonstrating why the ocean has distinct layers, why oil floats on water, and how density differences drive the ocean circulation that regulates Earth's climate.
Equipment Needed
- A tall, clear glass or glass vase (the taller the better)
- Honey (2-3 tablespoons)
- Washing-up liquid (2-3 tablespoons, a distinct colour from food colouring choices)
- Water
- Cooking oil (sunflower or vegetable oil)
- Rubbing alcohol or methylated spirits (available from hardware or DIY shops)
- Food colouring in several colours (red, blue, green, yellow)
- A dropper, syringe, or spoon for careful pouring
- Optional: small solid objects to test (coin, grape, dried pasta, berry, cherry tomato)
Where to source: Rubbing alcohol (isopropyl alcohol) or methylated spirits from hardware, DIY, or pharmacy. Ensure no young children consume any liquids.
Safety note: Rubbing alcohol and methylated spirits are flammable and should be kept away from flames. Not for consumption. Keep out of reach of young children.
Method
1. Prepare your layers (10 minutes)
You'll pour liquids in order from densest to least dense. The key is pouring each layer very slowly over the back of a spoon held just above the previous layer—this breaks the liquid's fall and prevents mixing. Patience here is everything.
Prepare your layers in separate cups and colour them distinctly:
- Layer 1 (bottom): Honey — no colouring needed, it has its own golden colour. Density: ~1.4 g/ml
- Layer 2: Washing-up liquid — often already coloured (green or blue). Density: ~1.1 g/ml
- Layer 3: Water coloured red with food colouring. Density: ~1.0 g/ml
- Layer 4: Cooking oil — no colouring needed, it's yellow. Density: ~0.9 g/ml
- Layer 5 (top): Rubbing alcohol coloured blue or purple. Density: ~0.79 g/ml
2. Pour with patience (15 minutes)
Pour honey into the glass first to form the bottom layer—about 2cm deep. Pour slowly down the side of the glass.
Hold a spoon just above the honey surface. Very slowly pour the washing-up liquid over the back of the spoon so it trickles down rather than splashing. You should see it float on the honey with a clear boundary between them.
Repeat for each layer in order, always pouring over the back of a spoon. Each liquid should sit clearly above the one below.
The oil and alcohol layers are particularly important to pour carefully—oil really does float above water, and alcohol really does float above oil, but it takes gentle handling to establish the layers without mixing.
Allow the glass to settle undisturbed for 5 minutes before viewing. You should have a beautiful multi-layer column—five distinct colours with clear boundaries.
3. Drop objects and observe (10 minutes)
Now for the most visually engaging part. Drop small objects into your column and watch where they settle:
- A grape settles between oil and water layers
- A small piece of dried pasta may settle between water and washing-up liquid
- A cherry tomato usually settles at the oil-water boundary
- A coin sinks to the bottom
- A small piece of candle wax floats between water and oil
Each object settles at the boundary between the liquid less dense than itself and the liquid denser than itself—it sinks through liquids lighter than it and floats on liquids heavier than it.
4. Discuss and analyse (5 minutes)
Try predicting where each object will settle before dropping it. Get everyone to make written predictions before observing. How accurate were predictions?
Expected Results
A successful density column is visually stunning—five distinct coloured layers with sharp boundaries between them. The objects settle at predictable levels corresponding to their own densities. The honey at the bottom appears golden; the washing-up liquid layer is typically green or blue; the red water sits above; the yellow oil floats above the water; the coloured alcohol forms the top layer.
The boundaries remain sharp for hours if undisturbed. Stir the column, and the layers gradually separate again as density differences reassert themselves—a slow, beautiful process to observe.
Troubleshooting: If layers mix, you're pouring too fast or from too great a height. Start over—pour the honey again and work up patiently. If the alcohol and oil mix rather than separate, ensure you're using a clear, light vegetable oil rather than dense olive oil, and that your alcohol is at least 70% concentration.
The Science Explained
Density is mass per unit volume—how much matter is packed into a given space. Denser liquids sink beneath less dense ones because gravity pulls everything downward, but the upward buoyancy force a liquid exerts on a submerged object equals the weight of fluid displaced. If an object is denser than the surrounding fluid, gravity exceeds buoyancy and it sinks. If less dense, buoyancy exceeds gravity and it rises. At equal density, it floats neutrally.
Your density column works because each liquid is distinctly less dense than the one below. The liquids don't mix because mixing requires work—energy to displace denser fluid upward against gravity. Left undisturbed, the system maintains its layered, minimum-energy configuration.
Ocean density stratification works exactly like your column but at global scale. The ocean has distinct layers: a warm, less dense surface layer; a thermocline transition zone; cold, denser deep water. These layers don't readily mix because the density difference creates a stable stratification. This layering has profound consequences for ocean chemistry—the deep ocean is oxygen-poor because photosynthesis only occurs in the sunlit surface, and the lack of mixing means oxygen doesn't reach depth.
The global ocean circulation (thermohaline circulation, or ocean conveyor belt) is driven by density differences. In the North Atlantic, warm surface water carries heat northward, cools, and becomes denser (both cold water and salt water are denser). This dense water sinks to the seafloor and flows southward as deep ocean current. This overturning circulation distributes heat globally and is crucial to Britain's relatively mild climate—without it, Britain at 50-60° north would have winters far more severe than it does. Climate scientists monitor this circulation carefully because freshwater from melting Greenland ice sheets could potentially disrupt the density-driven sinking, weakening the circulation.
Oil and water separation is fundamental in environmental science and industry. When oil spills at sea, it forms a surface slick precisely because crude oil is less dense than water. The environmental damage of oil spills—coating seabirds, blocking light penetration to marine life—directly results from this density difference. Oil dispersants used in response to spills (like those used after the Deepwater Horizon disaster) work by breaking oil into tiny droplets that disperse through the water column rather than floating as a surface slick.
Real-World Applications
Density differences are exploited throughout industry. Steel ships float because the ship's average density—hollow steel hull filled with air—is less than water, even though the steel itself is much denser. Submarines control their depth by adjusting ballast tanks—adding or removing water to change average density. Hot air balloons float because heated air is less dense than cold air.
Density differences are used to separate materials in recycling—different plastics have different densities, allowing separation by floating in solutions of specific densities. Mining uses density differences to separate valuable ore from waste rock.
Taking It Further
Variation 1: Make a density column using only water and salt. Highly saline water (several tablespoons of salt per cup) is significantly denser than fresh water. Colour two batches differently and layer them. How stable is this column? What happens when you warm the glass in your hands?
Variation 2: Investigate how temperature changes density. Heat some water and add identical amounts of food colouring to hot and cold water. Very carefully pour them one above the other—which floats? (Hot water is less dense and should float above cold—important for understanding why warm surface water caps the ocean.)
Variation 3: Challenge people to identify mystery liquids by their position in the column. Drop an unknown liquid into the column and estimate its density from where it settles.
Experiment 3: Oobleck — The Liquid That Acts Like a Solid
What You'll Learn
Make a non-Newtonian fluid that flows like a liquid when treated gently but turns solid-like when struck—exploring the strange physics of complex fluids and discovering why body armour, bulletproof vests, and the quicksand you've been watching in films all involve the same fascinating science.
Equipment Needed
- Cornflour (cornstarch) — 2 cups (300g approximately)
- Water — 1 cup (approximately)
- A large bowl or baking tray with raised edges
- Food colouring (optional)
- A tablespoon for mixing
- Old clothes or an apron — this gets messy
Safety note: Non-toxic and entirely safe. Very messy—protect surfaces and clothing. The mixture is hard to clean if it dries on surfaces; clean up whilst still wet.
Method
1. Mix your oobleck (5 minutes)
Pour 2 cups of cornflour into your large bowl. Slowly add water, about a quarter cup at a time, mixing constantly. The mixture will go through a crumbly stage before suddenly transforming into a smooth, strange substance that seems to be simultaneously liquid and solid.
The correct consistency is achieved when: the mixture flows like a thick liquid when you tip the bowl, but cracks and feels solid when you press your hand firmly against it. Adjust with small amounts of water (too thick) or cornflour (too runny) until you achieve this balance. Add food colouring if desired—mix it into the water before adding.
2. Explore the basic properties (10 minutes)
Dip your hand slowly into the oobleck. It yields easily—your hand sinks in as if into a liquid. Now withdraw your hand slowly. Easy. Now plunge your hand in fast. It resists—feeling almost solid. Pull your hand out fast: it clings, as if grabbing you.
Try rolling some oobleck into a ball in your hands. Roll quickly and it forms a firm, smooth ball. Stop and it melts, flowing through your fingers.
Punch the surface of the oobleck firmly. Your fist doesn't sink in; the surface "hardens" under impact. Now press a finger in very slowly and it sinks through without resistance.
3. Advanced demonstrations (15 minutes)
Running on oobleck: If you have a large container (a plastic storage box works) and make a large batch, people can try running across the surface on bare feet. Run quickly and your feet don't sink in; stop and you sink slowly. Walking slowly across is impossible—you immediately sink. This is genuinely extraordinary to experience.
The vibration test: Place your oobleck bowl on a subwoofer or hold a powered speaker against the bowl. Play a low bass frequency—50-100Hz works well. The oobleck begins behaving strangely: fingers of oobleck rise and writhe in response to the vibration, looking almost alive. This is cornstarch dancing—the vibrations create pressure waves in the fluid that produce the solidifying effect in rhythmic patterns.
The cornstarch speaker: Place a thin membrane (cling film or a latex glove stretched over the bowl) on a speaker, add oobleck on top, and play music. The oobleck dances and forms standing wave patterns—a dramatic and beautiful demonstration of acoustic physics.
4. Investigate the thresholds (10 minutes)
Systematically test: at what rate of movement does oobleck transition from liquid to solid behaviour? Use a wooden spoon, pushing at different speeds. Try to find the threshold speed. This is more scientific than it sounds—you're investigating the critical shear rate of a non-Newtonian fluid.
Expected Results
Oobleck behaves exactly as described—solid under rapid force or pressure, liquid when force is applied slowly. The running demonstration is the most dramatic: people genuinely run across the surface without sinking. The vibration demonstration produces startling visual effects that look computer-generated.
The sensation in your hands is extraordinary—rolling it into a ball and feeling it melt is one of those experiences that bypasses rational understanding and produces direct wonder.
Troubleshooting: If your oobleck is too runny (always liquid), add more cornflour a tablespoon at a time. If too thick (always solid), add water a teaspoon at a time. The correct ratio is approximately 2:1 cornflour to water by volume, but exact ratios vary with cornflour brand. If you can't get it to solidify when struck, try a slightly drier mix.
The Science Explained
Oobleck is a non-Newtonian fluid—a fluid that doesn't follow Newton's law of viscosity. Newton described how ordinary fluids (called Newtonian fluids) have viscosity that's constant regardless of how fast they're stirred or stressed. Water, oil, and honey are Newtonian—stir them faster and they resist proportionally, but the viscosity doesn't change.
Non-Newtonian fluids behave differently. Shear-thickening fluids (like oobleck) become more viscous under stress or rapid movement. Shear-thinning fluids become less viscous under stress—tomato ketchup is famously shear-thinning, which is why it flows suddenly once you've applied enough force (and why tap-the-bottom-of-the-bottle works).
In oobleck, the cornflour particles are suspended in water. Under low stress, particles can slide past each other, and the mixture flows like a liquid. Under high stress or rapid movement, particles are forced together faster than the water between them can move aside—they jam together, creating a rigid particle network that resists further deformation. This jamming transition is fast, reversible, and pressure-dependent: apply more force and it becomes more solid; relax the force and it flows again.
This behaviour—dilatancy (shear-thickening)—has extraordinary practical implications. Body armour using shear-thickening fluid (STF) rather than rigid plates has been developed specifically because of this property. STF-impregnated Kevlar fabric is flexible and comfortable to wear but hardens dramatically on impact, absorbing and distributing the energy of a bullet or blade. The fluid that makes your oobleck solid when punched makes body armour stop bullets.
Quicksand works somewhat similarly to oobleck but in reverse—it's a shear-thinning mixture of sand and water. Under slow, steady movement, sand particles hold together and the mixture supports weight. But rapid movement or vibration disrupts the particle structure, causing the water to lubricate the particles as they rearrange—the mixture suddenly flows like a very viscous liquid. The way to escape quicksand is to move very slowly—exactly the opposite of instinct and Hollywood advice.
Real-World Applications
Non-Newtonian fluid research has exploded in recent decades as engineers recognise its potential. Smart fluids that respond to stress, magnetic field, or electric field are being developed for shock absorbers, bulletproof vests, smart prosthetics, and medical applications.
Rheopectics are fluids that thicken when stirred continuously over time—like oobleck but the thickening is cumulative. These are used in some paints to prevent dripping.
Thixotropic fluids thin under sustained shear but recover viscosity when resting—these include many gels, creams, and industrial fluids designed to stay in place when undisturbed but flow when agitated. Yoghurt is thixotropic—stir it and it becomes more liquid; let it rest and it firms slightly.
Understanding non-Newtonian behaviour matters enormously in food science, cosmetics, pharmaceuticals, concrete engineering, and materials science. Many biological fluids—blood, mucus, cytoplasm—are non-Newtonian, and understanding their flow behaviour is essential for medical applications.
Taking It Further
Variation 1: Add iron filings or ferrofluid to oobleck and bring a strong magnet near it. The magnetic particles feel the magnet through the oobleck—creating a magnetically responsive non-Newtonian fluid that responds both to force and to magnetic fields.
Variation 2: Try making other non-Newtonian fluids. Thick, concentrated cornflour gravy is non-Newtonian. Concentrated ketchup is shear-thinning—how does it compare to oobleck? Toothpaste is also non-Newtonian: it holds its shape under low stress but flows when you squeeze.
Variation 3: Measure the transition. Use a pendulum of known mass swinging through oobleck—does the pendulum slow down more at higher speeds? Can you plot pendulum speed against resistance to quantify the non-Newtonian behaviour?
August Water Science: The Universal Substance
These three experiments illuminate different aspects of water's extraordinary properties—and they're connected. Surface tension is a consequence of water's hydrogen bonding network—the same molecular architecture that gives water its high density (relative to similar-sized molecules), its ability to dissolve salts (relevant to density columns), and its role as the medium in which biological molecules like cornflour proteins interact to produce non-Newtonian behaviour.
Water's properties aren't accidents. They arise from a single molecular feature: the water molecule's polarity and its ability to form hydrogen bonds. This one structural feature—a bent molecule with partial charges—gives water almost all its extraordinary properties. Higher specific heat capacity than almost any substance. Highest surface tension of common liquids. Expansion on freezing. Excellent solvent for polar and ionic compounds. The molecule is a masterclass in how simple structure produces complex consequences.
August is the month that makes water science most accessible and most enjoyable—warm enough for outdoor spills, light enough for long afternoon experiments, and blessed with the particular energy of summer that makes hands-on science irresistible. Get a bowl, get some cornflour, get some food colouring, and spend an afternoon discovering why the most common substance in your life is also the most remarkable.
