Convection is the transfer of heat in liquids and gases. It happens when warmer parts move and carry heat with them. Unlike conduction, heat spreads through the physical movement of the fluid itself.

Heating causes the liquid to expand, which lowers its density. The less-dense hot liquid rises, and cooler denser liquid sinks. This repeated rise-and-sink cycle forms convection currents that spread heat through the entire liquid.

Particles move from one place to another forming a continuous loop. The rising of warm fluid and sinking of cool fluid spreads heat throughout the liquid.
Common examples include boiling water, sea breeze (day), land breeze (night), and hot smoke rising above a flame.