Yesterday a reader and I indulged our very geekiest sides as we briefly discussed the way in which crystallizing molecules give off heat as they change phase from liquid to solid. But did you know the process works in reverse as well? Crystals that change phase from solid to liquid (which is to say, melt) actually absorb heat. This is known as an endothermic reaction and it’s what’s behind chocolate’s oft-touted ability to cool your mouth while you eat it. It’s not just a marketing ploy. It literally does it.