Although many people assume that heat simply rises, it actually travels from warmer areas to colder areas in one of three ways: conduction, convection, and radiation. Conduction transfers heat through physical contact, as warmer molecules shift their energy to colder ones. Convective heat transfer occurs in fluids (liquids and gases), including air. Warmer air is less dense than cooler air and rises above it continuously, creating ongoing convection currents. In radiant heat transfer, waves of heat energy travel through space from a warmer surface to a colder surface. Radiation occurs on a daily basis whenever the rays of the sun heat people or objects on the earth. Forced air and baseboard heating systems take advantage of convection currents, which allow rooms to heat up more quickly. Systems that include radiators or in-floor tubing (radiant heat) primarily use radiation, which warms surfaces rather than only the air.