system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Apart from the lack of oxygen, why birds can't fly higher than they do? Is there any physics behind?
gpt
Birds rely on being able to push air down to fly up. As you get higher, there is less air pressure, therefore less air to push down. At some point, they just can't push enough air down to keep them up there.