system
Given the following post, respond as if you're commenting on Reddit:
human
Why do commercial airliners fly at the altitude they do (≈35,000')? Could they potentially save $ lowering their average altitude slightly?
gpt
They fly at that altitude precisely *to* save money! 

In very (very!) simple terms, denser air creates more drag on a plane, but it also provides more lift. The most efficient use of fuel is to go higher, as the drag is substantially reduced. However, there's a limit to how a plane can fly based on the reduction in lift as you go up. 

To fly higher, you need either a lighter plane, one with bigger wings, or one that goes faster. An example of the first two is the U2 spy plane. An example of the second is the SR-71 blackbird. 

Passenger planes can't be all that light, and faster = more fuel = more expensive to run. They can fly a bit higher, but 35,000' is a general sweet spot for efficiency for the 747 class. The 787, for example, has a 43,000' service ceiling, and will regularly fly at 38,000' or more. Smaller passenger planes often have ceilings capped at 30,000', and will regularly fly lower if they aren't flying as far. Once again, there's a balance between the high altitude drag reduction and the fuel needed to get up there. For long flights, it's very much worth flying as high as the plane can go.