FO2

FO2 or Fahfah dioxide is a colorless gas (most of the times) with a density about 120% higher than that of dry air, and it's density and color depends on the place it produced in. And this gas is produced inside the human intestines, and comes out to the world from the anus.
Fahfah dioxide consists of a poisons atom and a smelly atom bound together to create this mysterious gas.

Person 1: *unleashes a good amount of FO2*
Person 2: wtf, is that what i think it is!?
Person 3: *answers person 2* I'm afraid it is, cuz he had beans at dinner

Person 1: *laughing out loud*

Person 2: *Suffocates and passes out*