Babies...

Adam walked into his parents bedroom to see his father on top of his mother.
"Don't hurt her daddy!" said Adam.
"It's OK Adam, we're trying to make a new baby sister for you."
Smiling, Adam retreated to his own room for the night.
The next day, the father returned home to see Adam crying his eyes out on the porch.
"What's wrong Adam?" the father asked.
Adam sobbed "After all your hard work to put my baby sister in mommy, the post man came today and gobbled her out!"

Found at
http://your-joking.atspace.co.uk/