The answer, says Honolulu Community College Professor Richard Brill, is elementary.
Heat is a form of energy and hot molecules are energetic molecules.
So, the more energy they have, the hotter the temperature. In conventional ovens, Brill says heat is transferred from the air to food by collisions of hot, energetic air molecules with colder, less energetic food molecules. This happens at the surface of the food. The inside of the food is then heated by conduction as the hotter, more energetic molecules near the surface collide with the colderless energetic molecules inside. Microwave ovens use an entirely different approach to heating food.