A nap :)

When Mary was pregnant, her 5 year old, Billy, was utterly amazed, and a little bit disbelieving, that his sister was growing in his mommy's tummy. So one day when the baby was especially active, she sked Billy to place his tiny hands on her tummy to feel the baby kick.

When he did, however, the baby was suddenly still.

"Oh, Billy, she must have decided to take a nap," shrugged Mary.

"A nap?" Billy marveled. "You mean there's a bed in there too?"