Non-polar molecules are hydrophobic (water-fearing), and polar molecules are hydrophilic (water-loving), because, as you remember, water is also polar. Soap molecules consist of both components: a non-polar tail and a polar head. Let's look at sodium stearate, a common soap molecule.
The hydrophobic tail binds to the non-polar oils on our skin; the hydrophilic head binds to water. In this manner, we can wash away the dirt and oily substances that build up on skin.
|The hydrophobic tails clump together because they want to keep water out. They don't like it. Like sticks to like!|
And I think that is the shortest, most to-the-point BoS post yet! For past Beauty of Science posts:
BoS #1: A Primer and Some Food for Thought
BoS #2: Antioxidants and Free Radicals
BoS #3: UV Radiation, Visible Light, Color
BoS #4: Sunscreens
BoS #5: Oils vs. Water (Polarity)!
Resources and other good reads:
ChemMatters Oct 2011: Demystifying Gross Stuff