We've all seen wildlife documentaries showing young animals—lion cubs, perhaps—wrestling, chasing, pouncing on their siblings. Observe household puppies and kittens and you'll see the same behavior: young animals at play.
Play is defined as spontaneous, energetic behavior with no apparent purpose or goal. But whenever there's considerable expenditure of energy, a closer look is warranted. There may not be apparent goals, but the true benefits of play are being recognized by a growing number of disciplines.