Like all amphibians, frogs are cold-blooded, meaning that their body temperature changes with the temperature of their surroundings. In winter, some frogs dig burrows underground or in the mud at the bottom of ponds. They hibernate in these burrows until spring.
Wood frogs are found in North America and some live north of the Arctic Circle. As the temperature drops below freezing, the wood frog drifts into a deep hibernation, its breathing and heartbeat grind to a halt, and as much as 65% of the water in its body gradually crystallizes into ice. Glucose in its blood acts as a kind of anti-freeze that concentrates in its vital organs, protecting them from damage, while the rest of the body freezes solid.