The National Science Fiction Light Bulb Joke

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I forget which convention it was -- maybe Boskone? -- where I was the toastmaster or emcee or such for the banquet and began by saying, "Please rise for the national science fiction light bulb joke."  But it went over well, and though I doubt you'll laugh at it here -- blogs are a terrible context for jokes -- if you've got a good delivery, you ought to be able to use it to wring a chuckle out of the unsuspecting.

Here's how it goes:

"How many science fiction characters does it take to change a light bulb?"
"I don't know.  How many?"
"Two.  One to change the bulb and the other to say, 'As you know, Fred, the light bulb was invented by Thomas Edison and operates under the principle of...'"

This joke was invented by the late Sarah Purdom, wife of science fiction writer Tom Purdom.  I've known a lot of clever people, but this is the only formal joke whose creator I've ever known.  Prior to hearing it for the first time, I always wondered where jokes came from.  Now I know.

They're made up by elegant, witty women.

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