Griddle cakes are made from a more floury, viscous batter than crêpes, popovers, and choux pastry, and can retain gas cells for the few minutes that it takes to cook them; so they rise on the hot pan surface and develop an aerated, tender structure. Pancakes may be leavened (and flavored) with yeast, or with whipped egg whites folded into the batter, or chemical leavening, or some combination of these. Russian blini sometimes include beer, which may contribute effervescence. The batter is poured onto the griddle and allowed to cook until bubbles begin to rise and break on the upper surface; the cook then flips the cake to set the second side before the leavening gas escapes.