Best glass industry joke ever!

A middle aged glass executive walks onto a plane, stashes his brief case in the overhead locker, and quietly sits down in preparation for a lengthy sleep on the flight. Next to him is a young double glaze salesman with a lap full of the latest electronic gadgetry. The DG salesman introduces himself to the glass executive, who would rather not meet anyone new, and would rather sleep.
"How about we play a game" says the DG salesman
"I'd really rather sleep" replies the glass exec.
Not put off the DG salesman states boldly "In this game you could make some good money"
With one eye open the glass exec responds with "Tell me more …."
"Well, here's how the game works. I ask you a question and if you don't know the answer you have to give me $5. Then you ask me a question and if I can't answer it I give you $5000."
With renewed interest the glass exec sits up and says "You start.."
"What's the distance in miles between the earth and the moon"
The glass exec has no idea and hands over $5, then looks the DG salesman in the eye and asks "What goes upstairs with 3 legs and comes downstairs with 4 legs?"
The DG salesman looks perplexed, then starts work on finding out the answer. He has his laptop open and is browsing the net, he's texting friends, he's using his satellite mobile to call people, and eventually after about 30 minutes he gives up. The DG salesman hands over $5000 in cash to the glass exec.
The glass exec thanks him, pockets and cash, closes his eyes and leans back to go to sleep.
"Please tell me the answer" cries the frustrated DG salesman.
"To what question?" responds the glass exec
"What goes upstairs with 3 legs and comes downstairs with 4 legs?" 
"I have no idea, Here's another $5" replies the glass exec.