Water and other liquids have a kind of invisible skin known as ‘surface tension’. This is a bit like the skin that forms on boiled milk or custard. It lets tiny creatures like pond skaters walk across the water without sinking into it. They weigh so little that the surface tension is strong enough to support them.
Surface tension may be great for pond skaters, but it does not help with washing us or our clothes. When we put dirty hands or dirty clothes in plain water, surface tension prevents a lot of the dirt from dissolving into it. Soap is useful because it lowers surface tension. It makes it easier for dirt particles to dissolve in water. So the water becomes dirtier as our hands or clothes get cleaner.
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