A cupola is a small, rounded dome that sits on top of a larger roof, often with windows or openings that let in light and air. You've probably seen cupolas crowning barns, churches, and government buildings. They look like little towers with curved tops, sometimes topped with a weathervane or decorative finial.

Farmers traditionally built cupolas on their barns for ventilation: the hot air from livestock rises up and escapes through the cupola's openings, drawing in cooler air from below. This simple design helped keep animals comfortable before electric fans existed. The dome on the U.S. Capitol building in Washington, D.C., is sometimes called a cupola and can be seen from miles away.

Architects add cupolas both for practical reasons and beauty: a barn looks more stately with a cupola on top, and a church cupola draws your eye upward. Some cupolas are tiny, barely larger than a birdhouse, while others are big enough to stand inside.