What is a convection current?

A convection current is the way that heat is transferred throughout a fluid. By fluid, we mean a liquid or a gas, where the molecules are free to move. Imagine a cup of water being heated from the bottom. The heat being supplied to the water increases the energy of the molecules at the bottom of the cup first. The water at the bottom becomes hotter and less dense as the molecules gain energy and spread out. The hot water then rises and the colder water that was at the top sinks to the heat source, as it is more dense. The hot water which has risen cools and the water that sinks is heated, and the process repeats itself. This movement of molecules due to heat transfer is known as a convection current.