system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens to a smell when it goes away?

Do the components of the gas settle on the ground? Do they remain in the air but decompose or combine to form compounds that aren't smelly? Do we just get used to the smell? Do they adsorb to the wall or permeate through the wall? Yes, I am wondering about farts.
gpt
First: What is a fart? 

A big chunk of 'em is methane, a very light gas that can easily blow away. There's other more complex organic scent molecules too, plus and depending on the type and strength of the fart, some vapourized liquids. (I am SO NOT GETTING INTO sharts. That's a whole different ballgame there, Sparky.). 

Well, now that we all know what a fart is, through enlightening definition OR unpleasant personal experience,  three things happen to its smell over time.

A lot of gas in that fart spreads out to the point that there's not enough of it for us to smell. The gas molecules don't go away, they just float around in the air in an ever-increasing volume until they're too weak for us to detect. And they last until those molecules are processed somehow into other molecules such as they fly into a candleflame and burn.

Some components get trapped into fabric. Fart naked on a sofa and that cushion will wear your essence for a while. And if the fart has liquid or oily bits in it, that fart stink can last a long time.

But the third reason the smell goes away fairly quickly is that we have a survival trait that only pays attention to "new" stuff, smells included, and we quickly erase the stuff that's been around a bit and recognized. Old stuff is not as threatening as anything new is. If someone farted three minutes ago and we squirt a little bit of wood smoke into the same area, people are going to instantly detect that smoky smell even though the fart's still around. Their brains filter out the fart smell after a while (unless it's overwhelming), just like we filter out kitchen smells when we're cooking but someone from another room still shouts "Hey what smells so good?".