Why doesn't a squirrel fall off the tree when climbing head first down the tree?

Squirrels have special hip, knee, and ankle joints that allow them to rotate the hind legs around so that they point up the tree as the squirrel heads down. Watch carefully, and you'll see that the squirrel is still gripping with its back claws as it climbs down. In addition to the above, if that is true, the squirrels have strong claws that can grip any surface. I once watched a squirrel climb the side of a brick house.