Matchstick boats

Build matchstick boats that will zip across a water surface - powered only by soap.

You will need:

• A matchstick
• A bowl or tray of clean water, which isn't soapy
• A knife
• A drop of washing-up liquid

What to do:

First, use the knife to make a split in the wooden end of the matchstick, making it into a Y shape.
Fill the bowl with water (just a cm or so is fine).
Gently float the matchstick on the water.
Place a tiny drop of washing-up liquid into the split on the matchstick.
The matchstick zips away from the washing-up liquid as soon as you drop the washing-up liquid on the water.

What’s going on?

Water molecules have an attractive force between them, so if you imagine that you're a water molecule right in the centre of a tank of water, you'd be pulled in every direction by all the water molecules around you - and all these forces would cancel out so you wouldn't move. But if you’re a water molecule at the surface then you won’t be pulled from the top at all because there is just air above you. So, you end up with a more dense film of molecules on top of the water. The result of this is surface tension – the molecules are more attracted to each other then to the air above. Water molecules are also attracted to other substances, so a matchstick, for example, will be pulled by the water wherever it is touching it.
Washing-up liquid is a surfactant, which is something that breaks down surface tension. So by adding washing up liquid, one side of the matchstick has the attraction between the water molecules broken, whereas on the other side of the matchstick, they're still attracting each other and the matchstick.
What the matchstick feels is a pulling force from all the molecules on the clean water side, but on the other side there is virtually no surface tension. So rather than being repelled by the washing-up liquid, it's actually being pulled from the other side, across the bowl of water. Clever eh?