Heat is the transfer of energy from a warmer to a cooler object. The transfer can occur by two different processes. If the hot object is in contact with a cooler one, the process is called conduction. The faster molecules in the hotter object strike the slower ones in the cooler one, transferring their thermal energy. The average speed, and thus kinetic energy of the molecules in the warmer object are reduced, while those in the cooler object are increased.

If the warm object is in contact with air or water, it heats the fluid. Hotter fluid has a lower density, and so it rises. Its place is taken by colder fluid, creating moving currents of the fluid, called convection currents. Convection is a very efficient way of transferring energy from hot to cold objects.

The thermal energy can also be transferred even if the warmer object isn’t in contact with any other object. The vibrations of the molecules create infrared electromagnetic waves that carry energy. These waves are called radiation. The warmer the object, the more energy in the waves. More energy goes from the warmer to the cooler object than in the reverse direction, so radiation cools the warmer object and warms the cooler one.