Here's a brain teaser that seems easy but is actually very hard:
We've got a cylinder split in half by a diaphragm. On the left side is some Helium gas at 0°C and 1 atmosphere of pressure. On the right side is a perfect vacuum. We're going to snap our fingers and make the diaphragm instantly disappear. The gas is now free to occupy twice as much volume, and it will quickly diffuse out.
What happens to the temperature?
Go ahead and actually write down a guess. (…)