Explain how the average speed of air molecules relates to the temperature of air. More speed means a higher temperature. Distinguish between temperature and heat. T emperature is the average kinetic energy of a collection of molecules. Heat is the transfer of energy from one substance to another.
Internal energy is the total kinetic energy and potential energy of a substance.
This includes rotational and orbital components. The Kelvin scale's zero point means absolutely no molecular motion, or, the absence of all kinetic energy. The Celsius scale has its zero point as the freezing point of water. Both scales have the same amount of energy change from one degree to the next. You can do meaningful math calculations only on an absolute scale such as the Kelvin.