system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does my dog shake his leg like he's "scratching the air" everytime I scratch his belly?
gpt
Sometimes something will go in your nose and it will bother you and make you sneeze.  Or if something comes fast towards your face your eyes will close to protect them.

The doggie sctaches the air for the same reason.  Part of the doggie's brain thinks that your fingers are something it needs to protect it from - a bug or something else that it wants to itch away.  So it makes the doggie's leg start moving.  But the doggie isn't really bothered, because your fingers are also scratching the itch (if the doggie really didn't like it, he could still move away or growl or try to bite you).