Why does the pepper run away?

Pepper floats on a bowl of water. Touch the water with a soapy finger and all the pepper shoots to the edge of the bowl.

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You need

A wide, shallow bowl of water (a white one shows the pepper best)
Ground black pepper
A little dish soap (one drop on a saucer)

Ask a grown-up to help. Ask a grown-up to help. Do not rub your eyes with peppery or soapy fingers, wash your hands afterwards, and do not drink the water.

Try it

Shake pepper over the water until the top is covered. It floats.
Touch the middle of the water with a clean finger. Nothing happens.
Guess first: what will the pepper do if there is soap on your finger?
Dip the tip of your finger into the drop of dish soap.
Touch the middle of the water with the soapy finger. The pepper runs to the edge!
For another try, pour the water away and start with fresh water: once soap is in the water, it does not work again.

Why did it happen?

The top of the water behaves like a thin, stretchy skin. That is called surface tension. Pepper is light and does not get wet easily, so it sits on that skin. Soap makes the skin weak right where it touches. All around, the skin is still strong, so it pulls away from the soapy spot and carries the pepper along to the edge. The pepper is not afraid of the soap, and the soap does not push it: the water pulls it away.

New science word: surface tension. It means the top of water holds together like a thin, stretchy skin.

Your challenge

Try it with fresh water and cinnamon. Will it run away, too?