Convection currents
Drop a few crystals of potassium permanganate to the bottom of a beaker of water and heat it: purple streams rise from the bottom and spread through the beaker.
Figure to be added
As the water at the bottom warms, its particles move apart, it becomes less dense and rises. Cooler water from elsewhere sinks to the bottom, warms, and rises in turn.
Convection
When a liquid or gas is heated it expands, becomes less dense and rises, and cooler liquid or gas sinks to take its place. Heat carried upwards in this way is convection, and the rising streams are convection currents.
A fire under a tree withers and scorches the leaves high above it because the air around the flame heats and rises.