Soft drinks are carbonated using pressurized CO₂ gas and a siphon that introduces the pressurized gas into the water or soda. The CO₂ gas is forced into the liquid at a concentration beyond that which could exist under atmospheric conditions and then the container is sealed, preventing it from being released. That’s why if you leave a glass of soda sitting out, it will go flat; the CO₂ escapes into the air.