Why does pâte à choux puff up?
A choux is an interesting batter that is created in a way that maximizes gluten structure. It is essentially cooked twice.  You combine and heat water and butter, then add flour and continue cooking the resulting paste. That paste forms a ball, and then several eggs are beaten in.  This batter is then piped onto a sheet and baked.  In addition to the gluten structure, the initial cooking also causes the starches to gelatinize, further reinforcing the structure.  In essence, you have created a deflated balloon.  When the piped shapes are baked, steam develops and inflates the "balloon."  It's all explained quite clearly right here.