Buoyancy is the upward force that water (or any liquid) pushes on objects, making them float or feel lighter. When you jump into a swimming pool, you don't sink straight to the bottom like a stone because the water pushes back up against your body. That upward push is buoyancy.

A Greek mathematician named Archimedes discovered why buoyancy works over 2,000 years ago. He realized that when you put something in water, it pushes aside, or displaces, some of that water. The water pushes back with a force equal to the weight of the water displaced. If an object is lighter than the water it pushes aside, it floats. If it's heavier, it sinks.

This is why enormous cruise ships made of heavy steel can float: their hollow hulls displace massive amounts of water, creating enough buoyancy to keep them afloat. It's also why a tiny rock sinks immediately: rocks are denser than water, so they don't displace enough water to create the buoyancy needed to float.

The word can also describe a cheerful, optimistic personality, as in “She has a buoyant spirit.” Just as buoyancy keeps objects from sinking in water, a buoyant attitude can keep someone from sinking into discouragement.