system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a barometer actually measure air pressure?

I've known it does for years but I never exactly knew how.
gpt
Inside a barometer is a sealed can, containing air. This makes it a bit like a metal balloon in that air is trapped, and can neither flow in or out of the can.

If the air pressure outside of the can changes, it'll squash the can slightly. If the air pressure drops, the can will expand. It's these changes in shape that drive the needle on a barometer.