A physicist is showing their friend the computer scientist a thermos flask. "You can put a cold drink in it and no matter how hot it is outside the drink stays

A physicist is showing their friend the computer scientist a thermos flask. "You can put a cold drink in it and no matter how hot it is outside the drink stays cold". The friend is suitably impressed. "But that's not all: you can put a hot drink in it and no matter how cold it is outside the drink stays hot!" Now the friend is puzzled. "But how does it know?" I long ago became depressed by the amount of code I've read that is written this way. Nowadays I just expect it.