Phyllo (or filo) dough, best known for its use in Greek and Turkish pastries like baklava, is sold in the freezer section of large grocery stores. It is an elastic dough pulled into very thin sheets and cut into large rectangles. Follow the instructions on the box for thawing. When working with phyllo, keep the unused dough sheets stacked under a piece of plastic wrap or a barely damp kitchen towel until you are ready to use them; otherwise, they may become brittle and tear easily. Frozen phyllo dough keeps well but not indefinitely; it becomes dry or sticky if stored for too long.