Imagine two 4-year-olds trying to play a simple game of catch. They can’t do it. Neither can throw the ball straight enough or catch well enough to make the game work. But a 4-year-old and a 9-year-old can play catch and enjoy it. The 9-year-old can lob the ball gently into the hands of the 4-year-old and can leap and dive to catch the other’s wild throw. In a world of just 4-year-olds catch is impossible, but in an age-mixed world everyone can learn and enjoy this game.