The Atlantic Scientists think it’s valuable, but they’re not quite sure why. Millennium Images / Gallery Stock Anyone who has ever chucked a tennis ball in the general vicinity of a border collie knows that some animals take play very seriously—the intense stare, the tremble of anticipation, the apparent joy with every bounce, all in pursuit of inedible prey that tastes like the backyard. Dogs are far from the only animals that devote considerable time and energy to play. Juvenile wasps wrestle with hive mates, otters toss rocks between their paws, and human children around the world go to great lengths to avoid make-believe lava on the living-room floor.