The Overqualified Plumber

A math professor is shocked by a plumber's bill and complains it's one-third of his salary. The plumber suggests, "Come work for us, the pay is triple. Just pretend you only finished 7th grade; they hate intellectuals." The professor gets the job and loves the money. Later, the company mandates an 8th-grade evening class. In the first math lesson, the teacher asks for the formula for the area of a circle. The professor goes to the board, forgets the formula, and tries to derive it using integrals, but keeps getting "negative pi r squared." He looks back at the class in panic.

The other plumbers whisper, "Switch the limits of the integral!"