Reducing The Fire

Two teachers, a physicist and a mathematician, were sitting in the
faculty lounge. Suddenly, the coffee machine sparked and caught on fire.
The physicist grabbed a bucket and leaped towards the sink, filled the
bucket with water and put out the fire.

Later that week the same two are siting in the lounge. Again, the coffee
machine catches on fire. This time, the mathematician stands up, grabs a
bucket and hands the bucket to the physicist, thus reducing the problem
to a previously solved one.