The quality especially to be desired in pastry is lightness, and this depends almost entirely upon the amount of cold air in the pastry when expansion takes place in the oven. The best pastry is therefore that which contains the greatest quantity of the coldest air prior to baking. The repeated foldings and rollings to which puff paste is subjected have this increase of air in view; while in short crust the expansion is aided by adding baking-powder, or other acid, and alkaline substances, which, when moistened, combine to form a constituent identical in its composition and effect with that of the atmospheric air to which puff entirely owes its lightness. The difference between puff, or flaky, and short crust is that in the former there are thin layers of air and pastry alternating, and in the latter the air fills small cavities all over the paste.