system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do airplanes oxygen masks have a bag that may or may not inflate? What would cause the bag to inflate/not inflate?
gpt
You breathe in, pause, breathe out, then pause before repeating when taking normal breaths. Even when you're anxious.

During the exhale and the pauses, the oxygen coming out of a normal oxygen mask is wasted. We don't care so much about this on the ground in a hospital because they tend to have oxygen generators and/or big reserves. Oxygen is literally in the air, we can concentrate it if we have heavy machinery.

The airplane can't afford to carry machinery large enough to concentrate oxygen for all passengers (and if the masks drop it's assumed SOMETHING is broken) so it's important to conserve. So while you exhale, the bag fills up. When you inhale, you breathe the "extra" air out of the bag. This way less oxygen is wasted and the plane can make do with a smaller/lighter oxygen supply.