|1. Pour enough water into a shallow dinner plate to just cover the bottom - you don't need more. Sprinkle fine ground black pepper on the water.|
|2. The water will look something like this.|
|3. Dip a clean toothpick into the water so it touches the bottom of the plate. Nothing happens... whoopy.|
4. Now dip the toothpick into detergent and touch the bottom of the plate again. Wowsers! This time, the pepper scatters away toward the edges of the plate in every direction. Cool!
What's going on?
The pepper doesn't contribute to the motion you saw but makes surface motion clearly visible. The motion results from the reduction in the water's surface tension when detergent is added.
Surface tension is the result of the strong attraction between molecules in a liquid. Water has an unusually high surface tension compared with most other liquids because water molecules are very strongly attracted to each other. This strong attraction allows you to slightly overfill a glass with water and some insects to skate on its surface.