Heat-sensitive mugs, also known as thermochromic mugs, contain a special type of paint or coating that reacts to changes in temperature. When hot liquid is poured into the mug, the heat causes the molecules in the coating to undergo a reversible colour change. This change reveals a hidden design or image on the mug.

The thermochromic material is often made from pigments or dyes that are sensitive to temperature variations. As the mug cools down, the molecules revert to their original state, and the design disappears again.