Coffee with Cream: Brain Teaser from Braingle

Question:
You are served a hot cup of coffee and room-temperature cream at a restaurant. You want to wait a few minutes before you drink the coffee, and you want it to be as hot as possible when you drink it. Should you pour the cream in the coffee:

a) Immediately
b) Just before you drink it
c) It doesn't matter

("don't add any cream" is not an option)

HEY! DON'T JUST LOOK AT THE ANSWER! THINK ABOUT IT!

Answer:
The driving force for heat transfer is temperature difference. The coffee by itself is very hot and will therefore cool down at a fast rate. Then once the cream is added the temperature will drop even more. If the cream is added immediately, then the temperature will drop initially but will then drop at a slower rate since the coffee with cream is cooler than the coffee alone and therefore the driving force for heat transfer is less. As an added bonus, adding the cream will increase the mass of the contents of the cup. A larger mass takes longer to cool down than a smaller one at the same temperature. Therefore, the cream should be added immediately.