There's a fairly widely known joke about boiling water (one version is below) that pokes fun at how mathematicians like to reduce new problems to known solutions. I've traced it back to a footnote on page 17 of Vilenkin's Combinatorial Mathematics for Recreation (Mir, 1972, trans. Yanovsky). Does anyone know an earlier source? Not very serious history of mathematics, I admit, but it would be nice to acknowledge who first came up with the witty story.
The story goes that a mathematician once asked a physicist: "You have an empty teakettle and an unlit gas range. How do you go about boiling water?" "Very simply," replied the physicist. "Fill the teakettle with water, light the gas and the put the water on to boil."
"Right," said the mathematician. "Now solve this problem: the gas is burning and the teakettle has water in it. How do you boil the water?"
"That's no problem at all," replied the physicist. "Just put the teakettle on the range."
"No," said the mathematician firmly. "You turn off the gas, pour out the water and we arrive at our first problem, which we know how to solve."
Now, when a new problem is reduced to an already solved one, we speak of applying the "teakettle principle".