Do an experiment. Touch it or break it. Generally, food is hot inside. When you heat milk in the morning, it always gets hotter and colder. The vibration and collision of water molecules in food are caused by high frequency electromagnetic waves. Yes But there must be water molecules to exchange places with it. External objects (such as glass, plastic, etc. ) contains no water molecules. . So heat is not generated by itself, but obtained through heat transfer. So it's cooler than inside. .