 Ethologists (students of animal, including human, behavior) point out that in play activities, young animals learn important skills they will need later on. Young lions, for example, play by going through actions that will be part of hunting. Humans have hunted with rocks and spears for tens of thousands of years, and so boys often play by throwing projectiles at targets. Marek Spinka (2001) observes that in playing, young animals move in exaggerated ways. Young monkeys leap not just from branch to branch, but from trees into rivers. Children not only run, but skip and do cartwheels. Spinka suggests that in play young animals are testing the limits of their speed, balance, and coordination. In doing so, they learn to cope with unexpected situations such as being chased by a new kind of predator.