Take two nails and scrape them together vigorously. Now touch the place you were scraping. It’s hot! Let them cool and try it again. It’s hot again! You could do this all day and the nails would never stop growing hot. If heat were a liquid, there would have to be an infinite amount of it even in a tiny nail. What’s the better answer? Heat is a measure of kinetic energy. Kinetic energy means the amount of motion the molecules in the nail have, or how much they jostle against each other. The hot spoon heats your hand not because something invisible is being poured in, but because the fast-jostling molecules of the spoon hit the molecules in your hand and start them jostling too. In the case of the nails, all that’s needed to produce more heat is to start jostling those molecules again after they’ve calmed down. You won’t ever run out of heat.