Why does a souffle increase in volume when it is baked?

A soufflé increases in volume when baked due to the formation of steam and the expansion of air bubbles trapped in the egg whites. When heated, the moisture in the soufflé turns to steam, which causes the mixture to rise. Additionally, the proteins in the egg whites coagulate and set, providing structure to support the soufflé's height. This combination of steam expansion and protein stabilization results in the characteristic puffiness of a soufflé.