Why is boiling water essential for choux pastry?

Boiling water is essential for making choux pastry because it causes the starch in the flour to gelatinize, which helps to give the pastry its characteristic texture. Gelatinization of the starch occurs when it is heated in water, causing it to absorb water and swell, forming a gel. This gel is what helps to create the structure and texture of choux pastry, and it also helps to keep the pastry from becoming too dense.