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Inscriptions on five-thousand-year-old clay cylinders from ancient Babylon say the stuff inside was made by boiling fats with ashes. No one knows when people first made soap -- maybe in an early hunting camp, when hot animal fats accidently mixed with the ashes from around a fire. Soap is made when fatty acids from grease, fish oil, olive oil or animal fats mix with what's called a strong alkali -- found in ashes.

Soap works because it lessens the surface tension of water. Surface tension is what makes water bead up into drops. Soap makes the water molecules less attracted to each other, so they spread out. Now the soap can go to work. Soap molecules have a structure that acts as a link between water and dirt particles. One end of the molecule is hydrophilic -- attracted to water -- and the other is hydrophobic -- attracted to substances that aren't water soluble. The soap molecules grab onto both the dirt and the water, so when the water washes away, the dirt goes with it.

Most "soaps" on the market today are really detergents. Their chemical structure is a bit different, and their fatty acids don't come from oils or animal fats, but from petroleum-based products. To see what's in a bar of soap, check the label.

Borrowed from Earth and Sky.