If one side has one, the other side has to get one. Once launched, they cannot be recalled. When they land, they screw up everything forever.
or
In the alphabed.
or
They don't lie.
or
Only one, but the bulb has got to really WANT to change.
or
The lettuce came in ahead, the egg got beat and the faucet is still running.
or
None. There never was any light bulb.
Notes: Probably the only really good light bulb joke of 1984.
or