Replace the question mark with a number

A mother and her very young son were flying Southwest Airlines from Kansas City to Chicago. The little boy (who had been looking out the window) turned to his mother and asked, "If big dogs have baby dogs, and big cats have baby cats, why don't big airplanes have baby airplanes?"
The mother (who couldn't think of an answer) told her son to ask the stewardess.
So the boy went down the aisle and asked the stewardess. The stewardess, who was very busy at the time, smiled and said, "Did your Mom tell you to ask me?"
The boy said, "yes she did."

"Well, then, you go and tell your mother that there are no baby airplanes because Southwest always pulls out on time. Have your Mom explain that to you."