Answer: Cream

In what would seem to be a rather counterintuitive way to keep your cup of coffee warmer longer, you could add a splash of cold cream. Despite the small volume of cream having a much lower temperature than the coffee, it actually helps stabilize the temperature of the coffee.

First, the higher the temperature of an object, in this case the volume of coffee in the cup, the faster it sheds heat into its surroundings. Adding the cream initially lowers the temperature of the coffee slightly (decreasing the amount of time the coffee spends at its hottest temperature where it sheds the most heat) and then further slows the release of heat as the fats in cream act as an insulator. As a result, you end up with a cup of coffee that stays warmer longer, even though it spends less time at its maximum temperature.