On a scale from one to ten, how obsessed with Harry Potter are you?
Q: On a scale from one to ten, how obsessed with Harry Potter are you?
A: About nine and three quarters.
The humor in this joke centers around the obscure fact that the students in the Harry Potter books catch a train from the platform nine and three quarters in order to get to hogwarts.
The person that is told this joke theoretically would know this fact, and thus find it funny because the answer to the joke shows that the jokester is probably in fact actually obsessed with Harry Potter.
This joke is moderately funny.