There are countably infinitely many of these jokes. Here is a representative sample.
Version 1: The arson version
A mathematician and a physicist were walking along during their lunch break when at a two-day convention when they realized they are going to be late for the afternoon session. “We’re going to be late,” says the physicist.
So the two begin to scramble back to their seats. However, no sooner do they start then they spy the engineering building on fire. Immediately, the physicist springs into action. He finds a nearby length of hose, jerry-rigs it to a nearby fire hydrant, and quickly puts the fire out. He then rushes into the building to make sure everyone is okay (they are). He then grabs the mathematician’s arm and rushes back to their seats. Amazingly, they make it on time.
The next day, the pair are again out walking about on their lunch break when they realize they are going to be late for the afternoon session.
Immediately the mathematician springs into action. He sets the nearby engineering building on fire, thus reducing the problem to one previously solved.
Version 2: Arson-free fire version
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.
Version 3: Fire-free version
A mathematician and an physicist are on desert island. They find two palm trees with one coconut each. The physicist shins up one tree, gets the coconut, eats. The mathematician shins up the other tree, gets the coconut, climbs the other tree and puts it there. “Now we’ve reduced it to a problem we know how to solve.”
Version 4: No reductions
An engineer, physicist, and a mathematician were playing cards in a parlor. A fire breaks out. The engineer starts to calculate how much water it takes to put out the fire. The physicist figures out the best theory on how to put out the fire. The mathematician tries to prove the fire doesn’t exist.