If a fart is gas & most times just comes out. How come some times it feels like a bubble rolling up the crack?

That means yer crack is nasty dirty, and you shouldn't talk about this in public, people won't want to hang with you no more.

A bubble is gas pretty much. Think of bubbles you blow (by mouth or otherwise) in water. Bubbles are filled with CO2. Air is less dense than water so it "floats" up to the surface. Ah you get the picture! lol

A fart is methane gas light a match to one sometime

Residue left from previous excretions leave oils, that is what you feel. The gas passing through the oils, so to speak.

Your backside sweats. Well all of you sweats, but sometimes due to stance or clothing parts of you don't evaporate fast enough to keep up.

I don't know the answer to this, but it's a great question!!

It's nasty, but a valid question.

I've sometimes wondered myself.

i would go with what russ told you

You have crap in your but!