Charles’s law is part of the story, but not the whole story—it accounts for about a quarter of the typical soufflé rise. The rest comes from the continuous evaporation of water from the bubble walls into the bubbles. As portions of the soufflé approach the boiling point, more liquid water becomes water vapor and adds to the quantity of gas molecules in the bubbles, which increases the pressure on the bubble walls, which causes the walls to stretch and the bubbles to expand.