From David Fisher,
Your Guide to Candle & Soap Making.
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Ever seen a water strider skating across the top of a still pond? How does it do it? Why doesn't it sink?

Surface tension.

While we don't normally think of water as "sticky", it actually does have some "stick" to it. The molecules of the water are attracted to each other and form a kind of elastic layer/barrier that allows the strider to float. It's also what keeps bubbles and water droplets together in a spherical shape...and is one of the physical properties of soap that helps soap clean.

Soap is a "surfactant" which is a contraction of "surface active agent." Surfactants are amphiphilic, meaning they contain both hydrophobic (water repelling) parts and hydrophilic (water attracting) parts. Essentially, it acts to make the water "wetter" - breaking the surface tension of the water, and allowing the water to clean the dirt and/or oil better. This simple (and fun I might add) experiment demonstrates how this works.

Take a piece of card stock or poster board and cut it into a boat shape like this, with a small notch cut out of the rear.