Aer Lingus has the Answer . . .

A mother and her son were flying Aer Lingus from Dublin to Heathrow.

The son (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 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 responded, "Did your mother tell you to ask me?"
The boy said, "Yes she did."
"Well, then, tell your mother that there are no baby planes because Aer Lingus always pulls out on time. Have your mother explain that to you."