Metals are particularly good conductors of heat because their particles are very closely packed so the vibrations are passed on very quickly. They also contain large numbers of free electrons. These drift slowly through the structure, giving metals their strength and other properties. As the metal is heated, the free electrons closest to the heat source are heated. This makes them move faster and they travel through the metal, colliding with both atoms and other electrons. This naturally makes them vibrate faster (or move through the metal faster in the case of collisions with other free electrons). Thus, the heat is passed quickly through the metal.