Convection is one of the three main types of heat transfer, the other two being conduction and radiation. Unlike the other two, convection can only happen in fluids -- liquids and gases. This is because the molecules have to be free to move.Heat energy can transfer by convection when there is a significant difference in temperature between two parts of a fluid. When this temperature difference exists, hot fluids rise and cold fluids sink, and then currents(movements) are created in the fluid. But why does this happen? Let's look at an example of convection in action to get a better understanding.