Heat—thermal energy—is actually mechanical movement, and in a solid with springy interatomic attachments like copper, that energy creates tiny whorls called phonons that move through the medium like a wave of sound does through air. When someone says that a metal like gold, silver, or copper is a good conductor of heat, what they’re really saying is that the metal is good at propagating phonons. And copper’s crystal structure, the way its atoms line up in bulk, is really good for working into new shapes—its atomic crystals are smoother on their faces than in other metals, so they slide across each other, metallurgically speaking.
Adam Rogers in Proof