Convection

Unlike conduction, where energy passes molecule-to-molecule while each particle stays near its own mean position, convection transfers heat by the actual BULK, BODILY movement of heated (energised) molecules within a fluid -- so it occurs only in liquids and gases, where molecules are free to move about.

Consider water being heated in a vessel from below: the water at the bottom heats first, its density drops (its molecules spread slightly farther apart), and this now-lighter hot water rises, while cooler, denser water from above sinks down to take its place. That newly-arrived water is then heated in turn and rises itself, while the water it displaces, now cooled, descends -- setting up a continuous circulating flow called a convection current, which carries …