Convection happens naturally because of gravity:
Pockets of denser fluid tend to fall because they are heavier while pockets of less dense fluid tend to rise because they are lighter.
- A pocket of fluid which is warmer than its surroundings expands and becomes less dense. The pocket begins to rise.
- The pocket is replaced by colder, denser fluid. This pocket of fluid is then also heated and starts to rise.
- As a pocket of fluid rises it begins to cool. It contracts and becomes denser again. The pocket starts to fall back down.
In this way, heat is transported throughout the fluid.
Remember that a fluid can be a liquid or a gas.
A radiator in a home heats the air around it. This hot air then rises and some colder air takes its place. The radiator then heats the new colder air and the cycle continues.
The radiator can heat all of the air in the room in this way.