Scientists say that "true" hibernators, such as ground squirrels, practically die during hibernation. Their body temperature drops to near freezing, and their organs shut down. A ground squirrel's heart rate, for example, slows from a few hundred beats per minute to just five. Even the little nut lover's bones and teeth deteriorate, as every tissue toes the line between life and death. They do wake up every few weeks. But normally, you can poke, prod, or even eat a hibernating critter and get no response.

Never try that with a bear. Yogi and Boo-Boo aren't exactly into winter sports, but they're not dead to the world either. Bears just use winter to catch up on their sleep, preferring winter dormancy to scrounging for dwindling food. Their body temperature drops only a few degrees. They wake up frequently and can stir in a second if someone invades their den. Mother bears even give birth to cubs and suckle them sleepily till spring.