system
Given the following post, respond as if you're commenting on Reddit:
human
What limits the height at which something can fly?

Birds, insects, planes, etc. all seem to have a glass ceiling as to how high they can go.  Why?
gpt
Air density.

The density of a ~~liquid~~ fluid relates directly to how easy it is to generate lift, so you need to fly faster. On the other hand, drag also decreases, but for propeller driven craft (such as prop planes, helicopters, and birds) you can be limited by the internal losses of the propeller system, plus you always have to fight gravity regardless of drag.

Air density also causes problems for anything that needs to breath oxygen. This obviously includes birds, but less obviously includes helicopters and planes. Both helicopters and planes work by burning fuel and oxygen. Spaceships must bring the oxygen with them, but planes and helicopters save weight by burning air scooped up from the nearby atmosphere (actually, your car does this too). When the atmosphere is less dense it is harder to pick up enough oxygen to power the engines.