What causes that? Is it the coffee cup that actually makes the coffee taste slightly cooler than room temp water in a glass? I'm guessing. I don't know if it's actually colder. But probably more due to the heat transfer you're feeling from your lips to the mug?

Either way, I'm going to go put this in the microwave so I can find it there again at around 10PM when I finally remember I didn't eat anything besides snacks and need to heat up a burrito.