Picture generated by Gemini.

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 then put the water on to boil."

"Right," said the mathematician. " Now solve this problem: the gas is burning and the teakettle has water. 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".