Surface tension can hold a surprising volume of water on a coin. If the surface tension is reduced, e.g. by adding a surfactant, the water spills over at a far lower volume.

Surfactants (e.g. detergents) tend to build a monomolecular film on a water surface. While a monomolecular layer is hard to observe with the naked eye, in this experiment it can be 'seen' pushing aside pepper on the water surface.