Biogas is gas made when plant material — and often animal waste too — rots in a closed place with little air. People catch that gas and burn it for cooking, heat, or electricity. On a farm it can come from dung and leftover plants; in a city, from food waste. The useful part is mostly the same kind of gas as in a kitchen stove, but it did not come from a hole in the ground. Though actually, raw biogas is not as clean as the gas in city pipes; it usually needs cleaning before it is nice to burn. It is a real biofuel, the gas kind, and a way of getting bioenergy. The word is a bit technical in English, but the idea is old and practical: rotting stuff gives off gas you can use. Do not picture the air we breathe; this is fuel gas.