At one time it was believed that heat was a liquid. Sure, that sounds kind of crazy, but if you think for a second, you might see why. Suppose you have an object that’s very hot — say a metal spoon that’s been sitting in hot coffee. Pick it up and you can feel the heat spreading to your hand. The longer you hold on, the more heat seems to enter your hand. Ouch!Well, maybe heat is an invisible liquid that pours from the spoon into your hand. After all, that theory fits all the evidence, doesn’t it?But there’s a problem. Another simple experiment you can do shows heat can’t be a liquid.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!