Methane is a simple gas. You cannot see it, and by itself you cannot smell it. It burns well, so it is a large part of the natural gas that cooks food and heats water — though gas companies add a stinky chemical on purpose, because pure methane would give no warning if it leaked. In nature it comes from wet ground, from the stomachs of cows, from rubbish dumps, and from under the earth. It is also a greenhouse gas: it traps heat in the air and adds to climate change. Chemists write it as CH₄ — one carbon and four hydrogens — the smallest of this family of simple burnable gases. Though actually, 'natural gas smells' is only half true: the smell is added. Methane itself is silent to the nose. Mixed with air and a spark, it can explode, which is why miners used to fear it in tunnels.