A physicist is showing a thermos to her friend, a programmer.
“It’s amazing”, she said. “You put a cold drink inside and regardless of how hot it is outside the drink stays cold”.
The programmer is suitably impressed.
“But that’s not all”, she continued. “You can put a hot drink inside and no matter how cold it is outside the drink stays hot”.
Now the programmer is perplexed.
Plaintively he asks, “But how does it know?”