A Professor of Mathematics and a Professor of Physics are in the staff room ...

A Professor of Mathematics and a Professor of Physics are in the staff room when the coffee machine bursts into flames. The Physics Professor jumps up, grabs the fire extinguisher and extinguishes the flame. The Mathematics Professor watches the whole thing without any reaction.

Years later, the same two professors are in the same staff room when again the coffee machine bursts into flames. This time, the Mathematics Professor jumps up, grabs the fire extinguisher, hands the extinguisher to the Physics Professor and says : I have reduced the given problem to a previously solved one.