Popovers are an American version of English Yorkshire pudding, which is cooked in the fat rendered from a beef roast. The batter is almost identical to crêpe batter, but a different cooking method transforms it into a large air pocket surrounded by a thin layer of pastry. Popover batter is vigorously beaten to incorporate air and cooked immediately, before the air bubbles have a chance to escape. The batter is poured into a preheated, liberally oiled pan and set in a hot oven. The surfaces of the batter set almost immediately. The air bubbles within the batter are trapped, expand with the rising temperature, coalesce into one large bubble, and the liquid batter balloons around it and sets into a thin blister. When cooked in a pan with many cups, popovers rise unevenly, because the cups around the outside heat faster than the ones in the middle of the pan.