Because there's no one else to wag it for him.
or
Billllyuns and billllyuns. Light bulbs are part of the interstellar "goo" that pervades our universe; they are star stuff.
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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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The barman says "I'll serve you, but don't start anything"
or