Wood frogs can tolerate the freezing of their blood and other tissues. Special proteins in their blood called nucleating proteins, cause the water to freeze first. This ice, in turn, sucks most of the water out of the frog’s cells. The frog’s liver makes large amounts of glucose to keep the cells inflated, like a natural form of antifreeze.
There is no beating heart or brain activity until they thaw, full flatline.
Humans lack these nucleating proteins, so when we freeze, we get frostbite, which lethally sucks all the water out of our cells and causes them to collapse. Put simply the frogs replace the water in their cells with sugar to enter a state of suspended animation.