Most animals would die if their bodies froze solid. The wood frog is different. This small brown frog lives in forests across Canada, Alaska and the northern United States, and it survives winter by freezing.
When cold weather comes, the wood frog hides under dead leaves on the forest floor. As ice begins to form on its skin, its liver starts pumping out large amounts of sugar. The sugar spreads through the frog's body and protects its cells, the tiny parts that make up every living thing.
Soon the frog's heart stops beating, and it stops breathing. Up to two-thirds of the water in its body turns to ice. The frog stays this way for weeks or even months.
In spring, the frog thaws from the inside out. Its heart starts beating again, and within a day it hops away to find a pond. Scientists study wood frogs to learn how human organs might one day be frozen and saved for medical use.