People in both North America and South America ate lots of sweet potatoes. That's because sweet potatoes are very good for you. They have lots of calcium, potassium, Vitamin A, and Vitamin C, and your body needs all of those things. Plus, sweet potatoes taste good, and they are very sweet, which helps to give you energy. Sweet potatoes are distantly related to morning glory flowers, and also to white potatoes.
Sweet potatoes are a kind of vine, with white flowers. The part you eat is the root of the vine plant. People first ate sweet potatoes by gathering them in the wild, where they grew naturally, which was in Central America and the warmest parts of South America. But sweet potatoes were so useful and so good that by about 3000 BC people had learned to grow them on purpose, and soon they shared that knowledge with their neighbors, and so after a while people were growing sweet potatoes all over South and North America, wherever it was warm enough for the plant to grow.
More sweet potatoes
When African people came to North America, they called sweet potatoes "yams". That's because back home in Africa, they had eaten a food that was a lot like sweet potatoes, which in Africa was called "nyami" or "anyinam". They aren't really the same plant,
but they look and taste a lot alike, so the African people called sweet potatoes "yams." African-American people cooked sweet potatoes the way they had cooked yams back home, and ate a lot of them.
To find out more about sweet potatoes, check out these books from Amazon.com or from your library:
Bread Comes to Life: A Garden of Wheat and a Loaf to Eat, by George Levenson (2004). From wheat to bread, lavishly illustrated, for kids.
Ancient Agriculture: From Foraging to Farming, by Michael and Mary Woods (2000). For middle schoolers, with plenty of information about how farming got started, and how it worked.