Nothing, it shuts them up!
or
None. Bill Gates will just redefine Darkness® as the industry standard.
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Because it would fall over if it lifted the other one.
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1,622. One to tell the original joke, and the rest to submit give some minor variation of it! For example:
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Twenty. One to hold the bulb, two to turn the ladder, and 17 to be on the guest list.
or