What is a dome?
Now suppose you want to make a dome. How would you do that using the idea of the arch?
This is harder, but basically a dome is an arch turned around and around. People did not get this idea until the time of the Roman Empire, around 100 AD. A famous early example of a dome is Hadrian's Pantheon in Rome. Another great dome from the ancient world is the one in Hagia Sophia, in Constantinople. Architects built Hagia Sophia under the Emperor Justinian in the 500s AD.
During the Islamic Empire, in West Asia, architects there built the Dome of the Rock mosque in Jerusalem in the 600s AD, and, in the 700s, the Great Mosque of Damascus, which also has a dome. In the 800s, the Abbasids built the Great Mosque at Kairouan in North Africa, also with a small dome.
Russian churches took over this style in the 1200s AD, and they have small onion-shaped domes.
To find out more about domes, check out these books from Amazon.com or from your library:
Arches to Zigzags: An Architecture ABC, by Michael J. Crosbie (2000). Shows what an arch is, or a gable, or an eave. For younger kids.
Eyewitness: Building, by Philip Wilkinson, Dave King, and Geoff Dann (2000). Lavishly illustrated, like other Eyewitness books for kids, and with good explanations of most architectural terms.
Dome: A Study in the History of Ideas (Princeton Monographs in Art and Archaeology, No 25), by E. Baldwin Smith (1985). By a specialist, for adults.