Soaked gram seeds in a test tube with a delivery tube into lime water — the lime water turns milky (CO₂ released); but no oxygen was supplied — anaerobic respiration demo. Idli/dosa batter rising overnight — yeast's anaerobic fermentation puffing it up. Your muscles' burn during a sprint — lactic-acid fermentation when oxygen can't keep up. Biogas plants run the same anaerobic logic at village scale.