For some time, little Joe had been a confirmed thumb-sucker, and his mother was unsuccessful in breaking him of the habit. Finally, she said to him "Joseph, if you continue to suck your thumb, your stomach will blow up bigger and bigger until it bursts. This did the trick because it scared the lad.
A little while later, Joseph's mother was entertaining some guests at home, and among them was a lady who was very, very pregnant. Her frontal contour, fascinated the boy who just stared at her.
Not being able to control himself any longer, Joe pointed at her and (in a voice that everyone could hear) said, "I know what you've been doing!"