Convection occurs in fluids. Fluids are liquids and gases.
When a fluid is heated it expands. The fluid
becomes less dense and rises.
The warm fluid is replaced by cooler,
denser fluid. The resulting convection current
transfers energy throughout the fluid.
Convection currents can be on a very small scale, such as heating
water in a beaker or on a very large scale, such as heating the air
above land and sea. Convection currents are responsible for onshore
and offshore breezes.