"Some of our TWA Coffee, or some of our..." joke

A mother and her son were flying TWA from Kansas to Chicago. The
son (who had been looking out the window) turned to his mother and
said, "If big dogs have baby dogs and big cats have baby cats, why
don't big planes have baby planes?"
The mother (who couldn't think
of an answer) told her son to ask the stewardess. So the boy asked the
stewardess, "If big dogs have baby dogs and big cats have baby cats, why
don't big planes have baby planes?"
The stewardess asked, "Did your mother
tell you to ask me?" He said that she had. So she said, "Tell your mother
that TWA always pulls out on time."