Question: How does a cow make a gas?

Ciarán O'Brien answered on 11 Nov 2014:

Cows eat grass, mostly. It’s a pretty high fibre food. Between the cow’s own digestion and the bacteria that live in its gut, it breaks down the grass into materials they can both use using a load of different chemical reactions.

But not everything the grass gets broken down into is useful as food. Some of the grass will get turned into a gas called methane, which is really quite smelly. The bacteria that do most of this are called methanogens, and they can be found everywhere there’s little or no oxygen, like deep in the ocean floor, or a cow or human’s gut.

These methanogens are the things that make the gas, and the poor cow takes the blame for it. Pro-tip: Next time you break wind, you can say it was the methanogens’ fault, not you!