A physicist and a mathematician setting in a faculty lounge.
Suddenly, the coffee machine catches on fire. The physicist grabs a
bucket and leaps towards the sink, fills the bucket with water and
puts out the fire. The second day, the same two sit in the same
lounge. Again, the coffee machine catches on fire. This time, the
mathematician stands up, gets a bucket, hands the bucket to the
physicist, thus reducing the problem to a previously solved one.