Illustrating by way of example, a single molecule of gas moving at a constant velocity will
have a definite temperature. Two or three molecules moving at the same speed will have the same
temperature, but together represent a greater thermal energy than any one of them considered alone.
Heat is either the reduction or increase of thermal energy by transfer of energy. If these gas molecules
happen to collide with slower-moving gas molecules, the faster molecules will lose velocity while the
slower molecules will gain velocity. Thus, the higher-temperature molecules cool down while the
lower-temperature molecules warm up: a transfer of heat.