Difficult Assignments

As a student, whenever I received an obviously challenging assignment, my first assumption was that the professor was a sadist who just enjoyed seeing students fail. (Apparently I'm not the only one; typing "sadistic professor" into Google gives 755,000 hits.)

Being on the other side, I do give challenging assignments in my classes, and even difficult questions on quizzes and exams. But I do it for the opposite reason: it doesn't give me any joy to see a student fail (quite the contrary), but it does give me great pleasure to see a student encounter a really tough problem... and then they succeed.