As told by the late, great Irish comedian, Dave Allen.

A 6 year old boy is sitting on a bus with his mother and starts sucking his thumb.

His mother tells him "you know what will happen if you keep sucking your thumb"?

"No" replies the boy

"You will get big and fat" his mother tells him

A few minutes later the bus stops and a heavily pregnant woman boards and sits opposite the boy and his mother.

The boy points at the woman and says...."I know what you've been doing"