Start with two dry pennies on a flat surface.

Using a dropper, measure how many drops of water one of the pennies will hold. Then add dish soap to some water and repeat the experiment with the other penny. You will find that the penny with plain water holds many more drops due to higher surface tension. When soap is added to the water the surface tension is lowered and the water breaks up much more easily. This breaking up of the water molecules is what allows soap to clean clothing and dishes.