A flatogen is a substance that increases gas production in the intestine — the flatulence-promoting factor in food. The classic flatogens are the oligosaccharides raffinose and stachyose in beans and pulses: human enzymes cannot digest them, so they arrive intact in the colon, where bacteria ferment them enthusiastically into hydrogen, methane, and carbon dioxide. Cabbage, onions, and Jerusalem artichokes carry their own flatogenic loads. The gas itself is harmless — socially awkward but physiologically normal — and the fermentation feeds beneficial gut bacteria, which is why the highest-flatogen foods are among the healthiest. Soaking beans, changing cooking water, and gradual dietary adaptation reduce the effect. The flatogen is nutrition’s comic villain: the price of fibre, paid in wind.