This joke reminds me of a scene from Star Trek: The Next Generation. Capt. Picard screaming, "There are four Lights!"
A student named Kevin was having an oral exam in philosophy with his professor. Kevin didn't study for the exam and couldn't answer any of the questions, even the easiest ones. The professor felt pity for him and gave him one last chance:
"Ok Kevin, if you can answer this question you will pass, otherwise you will have to retake the exam. Tell me how many light bulbs there are in this room?"
Kevin knew luck had smiled upon him and started counting the light bulbs on the ceiling.
"There are four light bulbs!", said Kevin, confidently.
The professor smiled, pulled out a light bulb from his bag and said: "Wrong, see you next time!".
Come the next exam, as usual, Kevin was still dumbfounded by all the questions. The professor once again asked the same question:
"How many light bulbs are there in this room?"
This time Kevin counted carefully and answered sheepishly:
"There are five light bulbs?"
The professor smiled once again:
"Wrong! I didn't bring a light bulb today."
Immediately Kevin reached into his pocket and pulled out a small light bulb.
"I brought one!"