Knock Knock Joke

My son finally make me laugh with his knock, knock joke.  While dressing for school he put on everything but his shirt.  When attempting to put on his shirt one arm came through and he left the other in the shirt.  He then said knock knock using his hand to knock from the inside.  I said, "Whose there", he said "hand", I said "hand who", he said "Hand wants to come out, hand wants to come out" in a really funny voice.  Its one of those things that you have to be there and  hear him.  It was so hilarious.  So proud he finally made a joke.  I always laugh at his other jokes but this one truly made me laugh so hard I cried.