Baking stones or tiles absorb moisture and produce an intense, dry oven heat that crisps pizza crusts or free-form bread loaves. Stones are porous and sometimes made of the same clay that lines commercial pottery kilns, while tiles are usually of high-fired terra-cotta. Stones are available in circular or rectangular shapes, large and small sizes; tiles are sometimes neatly contained in a metal tray. Preheat the stone or tiles on the bottom of the oven or on the lowest rack, then slide the pizza on top and bake. As an alternative, shape and bake pizza on a baking sheet.