Two. One to take out the bulb and drop it, and the other to try and sell it before it crashes (knowing that it's already burned out).
or
They turn from notating nuclear spins to notating unclear puns.
or
Two — one to change the bulb, and one to write a song about how good the old light bulb was.
or
At the Whale-weigh station.
or
By carving a Trumpkin.
or