Two. One to change the bulb, and one to write a song about how good the old light bulb was.
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Does it have to be a light bulb? I've got this neat candle holder...
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Nothing, peanuts don't talk.
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A conversation.
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A river.
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None. There never was any light bulb.
Notes: Probably the only really good light bulb joke of 1984.
or