Lightbulb and Basement

The joke plays on the classic 'how many people does it take to screw in a lightbulb' format, but with a dark twist. Instead of a silly answer, it implies that many children were put in the basement in an attempt to change a lightbulb, and this was unsuccessful, so they must still be there, or something has happened to them. It's meant to be a very dark and unexpected punchline.