Wood chips fly as this red-bellied woodpecker prepares its nest in the trunk of a tree. With each peck, a woodpecker’s head decelerates at up to 400 g, the standard acceleration due to gravity. That deceleration far exceeds what would render a concussive blow to a human brain. But with its much smaller size, a woodpecker’s brain can tolerate stronger shocks. For more on how woodpeckers avoid head injuries, see the Quick Study by Sam Van Wassenbergh and Maja Mielke on page 54 . (Image by FloridaStock/Shutterstock.com.)