Student: "Why do we have to clean up the floor before we leave?"
Me: "How did the floor get dirty?"
Student: "Isn't that the custodian's job?  Isn't that what he gets paid for?"
Me: "What a valid question.  How about you job shadow our custodian tomorrow and get back to me on your thoughts of whether or not we should clean up after ourselves?"
Student: "It's okay.  I'll clean the floor."