There's something excruciating about those few minutes of patience you need to muster before sipping that morning coffee you need to kick off the day. Drink it right away, and you're sure to burn your tongue. Wait too long, and it's bound to get cold. But there are a number of methods for cooling it enough to drink almost immediately.
Wired recently did a deep dive into the physics behind cooling a cup of coffee. The writer's experiment took two methods of cooling coffee—No.1, occasionally blowing over the tiny hole in the lid; and No. 2, taking the lid off completely—and tested them alongside a control cup of coffee (complete with a lid and no blowing action).
The three cups of coffee each had a thermometer placed inside to keep track of which one cooled the quickest. As it turned out, taking the lid off completely brought the quickest cool-down (Wired has a cool graph comparing the cooling times) by a landslide.
The science behind this is called "cooling by evaporation." This means that by taking off the lid of the the coffee cup, there's more of a surface area for the water on top of the coffee to evaporate—this is why you'll sometimes see water droplets on the inside of your lidded coffee cup after leaving it sit for a while.
If you need a caffeine kick ASAP but don't want a burned mouth, take the lid off and wait a few minutes.