A pun, a play on words, and a limerick walk into a bar...
No joke.
A father is washing his car with his son.
All of a sudden, the tired boy says, "Dad?"
"Yeah, son?"
"Can't you just use a sponge like everyone else?"
Don’t challenge Death to a pillow fight...
Don't do it unless you are ready for the reaper cushions!
There's a fine line between a numerator and a denominator...
And only a fraction of people will get this clean joke.