Why does water sit on a penny instead of spilling over the side?

Answer 1

We cannot put an unlimited amount of coins in a glass of water without it spilling over because the pennies displace their volume in water for each one put in. The space taken up by a penny, its volume, pushes water out of the way. The water must go somewhere as it can't be compressed, so the level in the glass rises and will, eventually (if the glass is full enough to begin with) overtop the glass and run down the side. In contrast, by adding something like table salt or table sugar to the glass in a gradual way, the level will not rise as the added material will dissolve and go into solution in the water (which acts as a solvent). The salt or sugar, either of which are called a solute in this case, will continue to be dissolved as it is added without increasing the level of the water in the glass until the solution becomes saturated with the salt or sugar. Addition of more material will then cause an increase in the water level as the saturated solution will not permit additional salt or sugar to have a "place to go" within the water molecules.

Answer 2

It's because of surface tension.

Water molecules cohere (are attracted to) all of their surrounding molecules. Molecules on the surface of water aren't attracted to molecules above it because there aren't any. This creates a net force that pulls them inward. This creates a dome shape on the penny. And when you add more drops and the gravity becomes stronger than the attraction, it spills.