The teacher had given the class an assignment. 

He stresses the importance of this particular assignment, and that no excuses will be accepted except illness (with a medical certificate) or a death in the immediate family (with a note from that member). 

A smart-ass student pipes up, "What about extreme sexual exhaustion, sir?" 

The class breaks up laughing, and when they settle down the teacher responds, "Well, I guess you'll have to learn to write with your other hand."