We got a cardboard boat to float on the water. The top of the water has surface tension on it. It is like a skin made by water molecules holding tightly on to one another.
The boat floated, but didn't move. When we put just one drop of washing up liquid at the back of the boat, the surface tension is broken and the boat shoots forward. 
This was really impressive, but we could only do it once. (Actually, we did manage to try again, but only after the basin had been washed and dried to get rid of all traces of wash up liquid). We can't wait to try this in the bath.
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