When it's going cheep!
Did you hear the one about the crow and the telephone pole?
He wanted to make a long distance caw.
or
They screw millions of bulbs every day, but when it comes to your bulbs, there's no record.
or
Only one, but she/he'll have to go out and buy the light bulb adaptor card first, which is extra.
or
Punch.
or
Because there are too many ears.
or