Well, for example, say Mommy and Daddy bought you this cool new toy that they call the baby ipod:

You love the baby ipod. When Mommy put you in your highchair, you accidentally on purpose threw said ipod across the room. Mommy picked it up, cleaned it off and gave it back. Hmmm.
47 times later, you have figured out that anything you throw, Mommy will clean and retrieve. Mommy figured out, that one backache later, it was time to move on to a new activity.