If you are the cat, press "B" to get your ball. Press "Q" to throw if you are the cat. If you are Nano, press the left arrow to get your ball. Press "space" to throw if you are Nano. Nano actually has a ball but you kind of can't see it. But if you move, you will see a blue ball. That is Nano's ball. The cat's ball is green.
look inside
