What makes a popover rise?

The secret to popovers' rise is in their unique batter, which is made with a high proportion of eggs and milk to flour and only lightly beaten. This ensures that the batter traps a lot of air, which expands when the popovers are baked, causing them to rise and puff up. The water in the batter turns to steam as the popovers bake, which further increases the pressure inside and causes them to rise even higher.