Either you a fart smeller, or a smart feller.

when we say "Either you a fart smeller or a smart feller" we say this because when you're a fart smeller, it means you're acting like nonsense, or dumb. When we say smart feller, you're thinking things through.

Guy: I can't get through this problem on the test, can you help me?
Guy 2: Either you a fart smeller, or a smart feller.