Knock Knock \ Who’s there? \ Dumbbell.
This is a slightly less lame knock-knock joke than is usual. That being said, it is still not that good.
Dumbbell doesn’t work so I had to knock!
The critical element of the joke that makes it funny is the pun centered around the word dumbbell. It is possible, however unlikely, that someone's nickname is dumbbell. Why the person would call themself that is left as an exercise to the reader, however, if a person were named dumbbell, then this conversation starts out normal. The last line, however, makes the listener change their original understanding of the word dumbbell to make it instead mean "[the] unintelligent bell."