system
Given the following post, respond as if you're commenting on Reddit:
human
If the Flash can run on water, how fast would he need to run or move his feet in order to run on air?
gpt
The problem isnt air density, it's the fact that he is surrounded by air.  When running on water, his foot hits the water surface but his leg rises through air.  The difference in density between the two is what allows him to apply more downward force than upward.  Trying to run on air, though, his leg would go up and down through the exact same fluid, meaning he would apply the same force in both directions.

You also mention surface tension of air,but air doesn't have a surface.