Sprinkle some pepper in a bowl of water. Most of it should float on top of the water, but if some goes to the bottom, don't worry.
Put a little liquid soap on your finger.
Lightly touch the top of the water and watch what happens.
Why did the pepper move? The attraction between water particles at the water's surface produces surface tension. This gives the surface a skin-like quality. Think of this thin skin-like quality like a balloon. Adding liquid soap is like popping the balloon. You break the surface tension; the thin "skin" breaks and pulls away to the sides. If there's something floating on these water particles, like the pepper, it goes along for the ride. For another fun activity that demonstrates surface tension, check out Soap Powered Boat.