Three - one to cast the bulb into the outer darkness, and two to catch it when it falls.
or
It takes two. One to change the bulb, and the other to kick the switch.
or
Big holes all over Australia!
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Four. One to change the bulb and three to whine "It's too high"
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A newspaper!
or
They turn from notating nuclear spins to notating unclear puns.
or