You can float a needle on water, if you are careful, but if you add a few drops of soap, that floating needle, will sink. The surface tension of the water forms a cohesive surface, and soap is a surfactant (wetting agent) that breaks this surface tension. That is why we use it to wash things. It helps the water to combine with what we wash off.

The fruit flies use this, to land on the surface of the cider, and tank up before they fly off. No youtube or anything, just physics. A lot of fruit flies were surprised when the surface refused to support them! :) :)