One is a blood-sucking parasite, the other is an insect.
or
A traffic jam.
or
None. There never was any light bulb.
Notes: Probably the only really good light bulb joke of 1984.
or
When it came time to pay, the skunk didn't have a scent, the deer didn't have a buck so they put the meal on the duck's bill.
or
Do you swear to pull the tooth, the whole tooth and nothing but the tooth?
or