Because it might crack up!
or
Two. One to take out the bulb and drop it, and the other to try and sell it before it crashes.
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To get to the shell station.
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There are footprints in the butter.
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Two. One to change the bulb, and one to write a song about how good the old light bulb was.
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Seven. One to change it and the other six to sing about how good the old one was.
or