About now, under the leaf litter in the woods of Alaska and Canada, wood frogs are getting ready to freeze. Not to get cold: to freeze, with something like two-thirds of the water in them turning to ice, the breathing stopping, the heart stopping. What saves them is sugar. As the ice starts at the skin, the liver pours glucose into the blood, and that keeps the inside of each cell liquid while the ice gathers in the spaces between them. In spring they thaw, and the heart starts again. The whole trick is that the frog doesn’t try to stay warm. It lets the cold in, and decides only where the ice may go.