Play for exercise and game unites us with other animal. We play when we are just puppies but we maintain the playfulness even when we come old. Children, dogs and cats understand immediately the joy of bouncing or rolling balls. Dogs and cats maybe because of their hunter's instincts, human child maybe just because it's fun. We like to roll around any round object to see it spin and move. Movement is in our soul and guts, and it makes us go!