Sprinkle some baby powder on top of the water. It will just stay on the top. In fact, you could try to mix it in, and it would still just want to stay at the top. The surface tension of water is such that it just holds those small baby powder particles on the top.
Now place a drop of dish detergent (or other soap) into the water....
Snowstorm!!
Soap breaks the surface tension of water (which is what makes it useful for cleaning) allowing the baby powder to fall through the water. It's interesting to watch - the powder will just fall through the hole made by the soap - the rest of it will continue to sit on top of the water.