I'm looking for a riddle about Oxygen

I recall seeing (many years ago) a riddle/puzzle that describes a substance that is SO lethal that the tiniest amount of it can kill you and yet it is all around us all the time—something like that but much more cleverly worded. The answer to the puzzle turns out to be Oxygen (because if you have JUST a tiny amount and no more, it will kill you – clever, eh?). Anyway, I’ve searched the web, but cannot find the original puzzle statement.