Be glad that you are not a math teacher:

A 51-year-old Malcolm X College instructor who was demonstrating a math problem on her blackboard Tuesday was stabbed in the back by a student who apparently became frustrated with the exercise …. [A] 40-year-old female student in the class repeatedly asked the instructor to explain a math problem and apparently became upset when she couldn’t grasp the material, Chicago Police said. The student pulled a small steak knife and stabbed the teacher in the left shoulder area, said Monroe District Capt. John Kenny. Kenny said that before the stabbing the student repeatedly said: "I don’t understand. . . . Explain to me.”

Annie Sweeney, Frustrated by Math, Student Stabs Her Teacher, Chicago Tribune.  (Hat Tip:  Chronicle of Higher Education.)