Temperature measures the average kinetic energy of the particles in matter: the hotter a substance, the faster its particles move. Thermal energy is that motion of atoms and molecules, and heat is thermal energy transferred from a warmer object to a cooler one until both reach the same temperature. When enough thermal energy is added, a solid melts and a liquid boils; when it is removed, a gas condenses and a liquid freezes. Pure water melts at 0 degrees Celsius and boils at 100 degrees Celsius at sea level, and its temperature holds steady while the state is changing.