Convection is a mode of heat transfer that occurs through the movement of a fluid (either a liquid or a gas). When a fluid is heated, it becomes less dense and rises, while cooler, denser fluid sinks, creating convection currents that transfer heat. Convection cannot occur in solids because the particles are fixed in position and cannot move freely. Convection also cannot occur in a vacuum as there is no medium (fluid) to transfer heat by bulk movement.