You've poured yourself a cup of coffee and then there's a knock on the door - the FedEx guy. You want your coffee to be as hot as possible when you come back in five or ten minutes. Should you add the milk now or wait until just before you drink it? Most people would say to wait, but that's wrong - put the milk in now. Look at the chart here. The blue line shows the temp when you add milk right away and the orange line shows the temp when you wait. The milk-less coffee stays hotter but you'll note that it is cooling a bit faster than the milky coffee. Then when you finally add the milk, at the red circle, it cools down to below the blue line. Why does it happen this way? Two reasons:

• The heat loss is proportional to the temperature differential between the cup of coffee and the room air. By adding the milk now, you have slightly cooled the coffee, reducing the temperature differential and the rate of heat loss.
• Dark-colored objects radiate more heat than light-colored ones, so the light coffee will radiate less heat than the black coffee and therefore cool more slowly.