Superman can fly, but a fly cannot superman!
or
Because they only know their ABCs "Anybody But Clinton".
or
Only one, but she/he'll have to go out and buy the light bulb adaptor card first, which is extra.
or
Because he was feeling crumby.
or
Two. One to assure the everything possible is being done while the other screws the bulb into the water faucet.
or