One; she designs the bulb to crawl up the wall, unscrew the old one, and screw itself in.
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Three. One to climb the ladder while the second kicks the ladder out from under her. And the third to say, "I knew that was too high for you dear."
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1,000,001: One to change the bulb and 1,000,000 to rebuild civilization to the point where they need light bulbs again.
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Just one, provided there is a programmer around to explain how to do it.
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