How to Walk on Water

Have you ever tried to walk on water? Chances are, you were unsuccessful (and no, ice skating doesn't really count). Why did you fail? Your density is much higher than that of water, so you sank. Yet, other organisms can walk on water. If you apply a bit of science, you can too. This is a terrific science project for kids of all ages.

Materials to Walk on Water

• 100 boxes cornstarch
• 10 gallons of water
• small plastic kiddie pool (or large plastic tub)

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How Oobleck Works

By Anne Marie Helmenstine, Ph.D.

What You Do

1. Go outside. Technically, you could perform this project in your bathtub, but there's an excellent chance you'd clog your pipes. Plus, this project gets messy fast.
2. Pour the corn starch into the pool.
3. Add the water. Mix it in and experiment with your "water". It's a good opportunity to experience what it's like to get stuck in quicksand (without the danger).
4. When you're done, you can allow the cornstarch to settle to the bottom of the pool, scoop it out, and throw it away. You can hose everyone off with water.

How It Works

If you trudge slowly across the water, you'll sink, yet if you walk briskly or run, you stay on top of the water. If you walk across the water and stop, you'll sink. If you try to yank your foot out of the water, it will get stuck, yet if you pull it out slowly, you'll escape.

What is happening? You've essentially made homemade quicksand or a giant pool of oobleck. Corn starch in water displays interesting properties. Under some conditions, it behaves as a liquid, while under other conditions, it acts as a solid. If you punch the mixture, it will be like hitting a wall, yet you can sink your hand or body into it like water. If you squeeze it, it feels firm, yet when you release the pressure, the fluid flows through your fingers.

A Newtonian fluid is one which maintains constant viscosity. Corn starch in water is a non-Newtonian fluid because its viscosity changes according to pressure or agitation. When you apply pressure to the mixture, you increase the viscosity, making it seem harder. Under lower pressure, the fluid is less viscous and flows more readily. Corn starch in water is a shear thickening fluid or dilatant fluid.

The opposite effect is seen with another common non-Newtonian fluid — ketchup. The viscosity of ketchup is reduced when it is disturbed, which is why it's easier to pour ketchup out of a bottle after you shake it up.