When Alex Rodriguez swings for the fences or Venus Williams tries to ace her serve, they do well to connect at the "sweet spot" of their bat or racket. That aim was apparently shared by some unlikely contenders: glyptodonts, armored mammals with clublike tails that roamed the Americas until about 10,000 years ago. The sweet spot, or center of percussion, is the point on a tool where powerful blows should be landed to maximize impact and minimize the risk of injury to the user.