Baby planes.

A mother and her young inquisitive son were flying British airways from London GW to Glasgow. The son (who had been looking out the window) turned to his mother and asked, "If dogs have baby dogs and cats have baby cats, why don't planes have baby planes?"
The mother (who couldn't think of an answer) told her son to ask the stewardess. So the boy dutifully asked the stewardess, "If dogs have baby dogs and cats have baby cats, why don't planes have baby planes?"
The stewardess responded, "Did your mother tell you to ask me that?" The little boy admitted that she did. "Well, then, tell your mother that there are no baby planes because British airways always pulls out on time. Now, let your mother explain that to you."