Convection is the mode of heat transfer in fluids, i.e., liquids and gases. Water gets heated in spite of being a poor conductor of heat due to convection. Water at the bottom of the pot gets heated when put on a flame. Hot water being lighter moves up and cold water moves down. The process of mixing continues until the whole mass of water has a constant temperature.
Land and sea breezes are caused due to convection currents in air. Water in a pot boils also due to convection.
Convection in liquids
Take a beaker of water. Put some crystals of potassium permanganate through a tube down at the bottom of the beaker. Now remove the tube and heat the beaker with over flame. The crystal colours the water so that you can see the convection current in the water. The warm (purple coloured) water rises to the surface. As it does, cooler water falls to take its place. This is because warm water has expanded, and so it is less dense than cold water. The coloured water falls down from the opposite side of the beaker as it is cool, forming a continuous flow or convection current.
Convection in gases
Take a rectangular wooden box having a sliding pane glass front, to make the interior visible. Two wide glass tubes are fitted in the holes at the top of the box as shown in the figure. Place a lighted candle below one of the glass tubes. Hold a smoking paper above the other glass tube. The air above the candle flame gets hot and rises. Cooler air is then drawn from the other tube to replace the hot air. Smoke is used so that you can see how the air moves. The movement of hot and cold air sets up convection current.