The aim of Catch 'Em is to catch all the falling objects without losing all your paddles. Each time you miss an object or you catch a lightning bolt, you will lose one of your paddles. As with any other game of this type, the higher the level you play the faster the objects fall. Also, you'll notice every few levels there is a bonus level to help give you more points. You don't have to catch any of the objects, but it certainly would be a good idea in order to boost your score.
