Rosie: "Daddy... How do you make potatoes?"
Daddy: "Well, they grow underground because they are a special sort of root. They are called 'tubers' actually."
Holly: "I know what a tuber is..."
Daddy: "Oh yes...?"
Holly: "Yes, a 'root' normally grows under the ground, but a 'tuber' is a special sort of a root that grows on top of the ground."

Heavens. That taught Daddy.