INCONSISTENCY THEOREM
LITTLE BOY: "My math teacher is crazy".
MOTHER: "Why?"
LITTLE BOY: "Yesterday she told us that five is 4+1; today she is telling us that five is 3 + 2."
ENGINEER AND MATHEMATICIAN --"USE OF STOVE"
An engineer and a mathematician shared an apartment. Their kitchen was equiped with an electric stove, and every morning someone had placed a pot of water on the back-right burner so they could make coffee. They both knew what knob turned on this burner. One morning the engineer came into the kitchen and found the pot was on the front-left burner. He got out the stove's schematics and followed the wiring diagram and finally figured out which knob turned on this burner and he then used that knob and made the coffee. The next moring the mathematician came in and also found the pot on the front-left burner. He moved the pot to the back-right burner, thereby reducing the problem to one which he had already solved.
* Both of the above jokes were cited from http://www.sonoma.edu/Math/faculty/falbo/jokes.html