Science Puzzle
Hot Tea, Cold Milk: When Should You Add the Milk?
You make two identical cups of tea. You must drink both in ten minutes, and you want them as warm as possible. You will add a splash of cold milk to each.
To end up with the warmest tea, should you add the milk straight away, or wait until just before you drink it?
The Answer
Add the milk straight away. It feels backwards, but the hotter an object is compared with the room, the faster it loses heat. A near-boiling cup haemorrhages warmth to the cool air around it; a slightly cooler cup loses heat more gently.
Splash the cold milk in at the start and you knock the tea down a few degrees immediately, which then slows its heat loss for the whole ten minutes. Leave the milk out and the piping-hot black tea races to shed heat the entire time, and only gets cooled by the milk right at the end. The early-milk cup wins, usually by a couple of degrees.
The principle: Newton's law of cooling. The rate an object loses heat is proportional to the temperature gap between it and its surroundings, so a bigger gap means faster cooling.