What you’ll need

  • Modeling clay in three different colors
  • A large container with water
  • Kitchen roll for drying the clay

How to do it

1. Take two pieces of clay and make a cylinder and a ball. Put it in the water. What happens?

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2. Make a small, flat bowl. Put it in the water. What happens?

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3. The small bowl floats!

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4. Place some small balls of clay in your dish. Test how much weight it can hold before sinking. Think about how you’d need to change the shape so that it can carry the weight of the balls again.

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Well spotted!

If you make a compact ball or sausage shape out of the clay, whether large or small, it sinks to the bottom of the container. But if you make the clay into an elongated trough, it floats, a bit like a soup bowl.

A cargo ship is the shape that works best: long and thin with high sides. You can even load it with more weight.

What’s the secret?

Clay is more dense than water and sinks if you shape it into a small cube or sphere.

During the experiment, you used the "buoyancy" effect of the water without realizing it. In other words, the power of the water to "push" from below against an immersing object, for example, a fallen leaf. This buoyancy is greater the more water the object displaces when immersed. Hollow, wide and long shapes are more buoyant because they have a larger surface area than compact shapes, such as a sphere.

This means that in addition to density, shape also plays a role in an object’s ability to float, because it can make use of the power of buoyancy.