Three women, a blonde, a brunette, and a redhead, die and are at the Pearly Gates. St. Peters tells them that if they want to enter Heaven, they need to walk up stairs that have one hundred steps and that on each step, they'll be told a joke. If they laugh, they're immediately sent to Hell.
So the brunette goes and on the third step, she cracks up because of the joke.
Then the redhead goes and is able to control herself until the thirteenth step, but then cracks up too.
It is the blonde's turn. She goes up every step and listens to the jokes without laughing. St. Peters is amazed and decides to give her a boring joke for the last step to see what will happen. So she hears the joke and cracks up.
Before sending her to Hell, St. Peters asks her why she laughed at the very last step with a joke so boring and she simply answers, "Well, I just understood the joke from the first step!!!"