What do green leaves, boiled eggs, and split gas molecules have in common? They have all undergone endothermic reactions, absorbing heat. But just because there’s heat involved doesn’t mean an endothermic reaction is the cause. Its polar — er thermal — opposite, exothermic describes a reaction that makes or gives off heat. It can be easy to confuse the two, but if you can remember that endo means “in” and exo means "out," the heat might not get to you so much.