Choux pastry is a twice-cooked pastry made from a dough of flour, water or milk, butter, and eggs, first formed into a panade on the hob, then enriched with beaten eggs and baked. In the oven, steam inflates the dough into hollow puffs that set crisp and golden, ready to fill with cream, custard, or savoury mixtures. It is the basis of profiteroles, éclairs, gougères, and Paris-Brest. Success depends on drying the panade properly so it absorbs enough egg, and on not opening the oven early, which collapses the shells. The hollow structure is pure steam engineering.