The scene: Thursday afternoon at geometry tutee's house.
Me: (looks over his shoulder as he works on a problem) Um, why are you subtracting?
Tutee: (blank stare) What?
Me: Let me ask you this - what theorem are you using to solve this problem?
Tutee: The Pythagorean.
Me: And the formula for the Pythagorean says....?
Tutee: a^2 + b^2 = c^2. (in an exasperated, condescending tone)
Me: Correct. Therefore, I go back to my first question - why are you subtracting the squares of the legs from each other?
Tutee: (stares at his handwriting) Oh. Heh.
Me: Dude, you must be super glad for the weekend.
Tutee: (sheepishly corrects his work) It's been a long day.
I always enjoy tutoring my geometry student. He's the smartest of the bunch that I tutor. He also has a great sense of humor. But sometimes, he gets a little annoyed/embarrassed, in that way a teenager gets, when I make corny math jokes. He and I both know he's laughing the hardest on the inside.