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This experiment works because of water’s surface tension. Water likes to stick to itself, and surface tension is a bit like a skin formed by the water molecules at the surface. The pepper is small and light enough that the surface tension can support it. You can feel the resistance of surface tension if you slap your hand onto the water in a bucket or bath – or belly-flop into a pool!

Soaps and detergents are both examples of surfactants, which are chemicals that reduce the surface tension of water. This is part of what helps them clean away oils and dirt from our hands and dishes.

When we touch the surface of our pepper water with the detergent, it reduces the surface tension in that one spot. This means that the surface tension is higher everywhere else, so the rest of the molecules at the surface pull back from that spot, stretching the ‘hole’ until the whole surface is affected. It’s a similar effect to popping a balloon. This movement between areas with different surface tensions is called the Marangoni effect.

The pepper in this demonstration helps us to see how those water molecules at the surface are moving. After the pepper has ‘pounced’, the lower surface tension let some of those bits of pepper sink to the bottom.

You may have noticed that you can only make the pepper ‘pounce’ once. If you want to repeat the experiment, you’ll need to rinse all the detergent off the plate and refill it with clean water.