Steam from heated water helps lift croissant dough during baking.
Croissant dough contains water in both the dough itself and the butter or other roll-in fat. In the oven, heat converts that water into water vapour, or steam. The steam expands rapidly and raises pressure between the laminated layers, helping separate them and contributing to the pastry’s flaky lift.
Steam is only one part of the process. Yeast fermentation has already expanded the dough during proofing, while the gluten network helps hold the gas bubbles. The fat layers act as barriers so the dough sheets can remain distinct instead of merging into a solid mass.
A common mix-up is to credit the oven’s air or a special gas such as helium with the rise. Neither is the key explanation. Croissant lift comes from the interaction of fermented dough, laminated fat, water vapour, and heat. If butter melts too early or the layers are damaged, steam can escape and the finished croissant may be flatter and less flaky.