When you are eating a girls ass out and she farts in your mouth. You then hold the fart in your mouth and pass it from your mouth to hers.
"Man last night this chick and I were fucking, but she farted in my mouth when I ate her ass out"
"Did you swallow it?"
"Nah, man I just did a methane exchange and gave it back to her."
"Nice, dude"
"Did you swallow it?"
"Nah, man I just did a methane exchange and gave it back to her."
"Nice, dude"
by Casual Rapist May 24, 2013