Closeup of a torus being eaten by ice cream.
(1) An O-ring is a gasket consisting of a flat ring of rubber or plastic shaped like a doughnut, but not really a doughnut. Only shaped like a doughnut. A little one. An overly-chewy little one.
It is used to seal a connection against high pressure gases or liquids.
So pressurized stoves use them. To seal connections against high pressure gases or liquids.
You can think of an O-ring as your personal ring of power. With it, your stove works and you can make supper. Without it you're only cold and hungry again. Your choice. Remain alert! Mind your torus, Horace!
(2) Also known as a packing, or a toric joint, an O-ring is a mechanical gasket in the shape of a torus, a doughnut, or if you lack imagination, the letter O. (Lions! and Tigers! Or Owes. Meh.)
This thingy is a loop of elastomer (rubbery stuff) made to be seated in a groove and squeezed in tight between two or more parts during assembly, creating a seal between them. And so it is used in pressurized stoves, of course. To seal its connections against high pressure gases or liquids.
The Dark Lord didn't need an O-Ring, but his stove did.
Come to think of it, he didn't need a stove either.
Never mind that then.
(3) An O-ring is a stove part that forms a seal, in stoves that need seals. These rings are usually made of some sort of flexible material, like silicone-based rubber. They are not needed in simple alcohol-burning stoves.
Like every other complex thing, your O-ring usually fails at the worst possible time, and can't be replaced by any old whatever that you might find lying around. So keep in mind how well O-rings served that space shuttle, then maybe take another look at alcohol stoves. You could do worse, mate. And the onus is back on you.
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Me? O Mama! Presently surrounded by rats, some of them educated.