The Potty

A little three year old boy is sitting on the toilet.
His mother thinks he has been in there too long, so she goes in to see what's up.
The little boy is sitting on the toilet reading a book. but about every 10 seconds or so he puts the book down, grips onto to the toilet seat with his left hand and hits himself on top of the head with his right hand.
His mother says: "Billy, are you all right? You've been in here for a while."
Billy says: "I'm fine, mommy. I just haven't gone 'doody' yet."
His mother says: "Ok, you can stay in there a few more minutes, but, Billy, why are you hitting yourself on the head?"
Billy says: "It works for ketchup."