Convection is the transfer of heat throughout a fluid by the movement of the particles that make up that fluid.

A fluid is anything that can flow - liquid or gas.

Many people explain convection by saying that 'heat rises'. Heat doesn't always rise.... it travels from hot to cold areas. We think that it does because of our experience of convection. Heated fluids rise! .... we experience this in our everyday lives and therefore the generalisation that heat always rises gets repeated.

When a fluid is heated it's molecules become exited and start vibrating more vigorously creating more space between them. They spread out. More space between the molecules means bigger volume, the mass stays the same so it has less density which means that it floats up in surrounding cooler denser fluid. The cooler fluid then replaces the warm rising fluid and a current is set up (see illustration above). So, where gravity is a factor, such as here on earth, the natural fluid motion moves the heated, less dense fluid up and the cooler, more compact fluid down. This is why heating vents are located along the floor, while air-conditioning outlets are located near the ceiling.

Convection is NOT POSSIBLE in a solid because the particles in a solid cannot move relative to one another. It is the principal mode of heat transfer within fluids.

The 'Chimney' Demonstration illustrates convection currents in gases.

Remember to always mention how it is difference in density that makes warm fluid rise and that cool fluid moves in to replace the displaced fluid - and to mention the term 'convection current' when you explain what happens....