What makes popovers “pop”
Popover batter is simple, with few ingredients and no chemical leaveners. So what makes them rise? While baking at a high temperature, the eggs in the batter create steam, and the gluten in the flour prevents that steam from escaping. The tall narrow cups of the popover pan force the popovers up and up, and the steam comes together to make one big bubble which remains after they’ve baked, giving them their characteristic hollow center.