From a discussion moments ago with the five-year-old:
Frank: “Look, there are some ducks flying over there.”
Stuart: “No, those are geese.”
Frank: “Whatever, same thing.”
Stuart: “No, they are different. You can tell them apart by neck size.”
Frank: “How’s that?”
Stuart: “Well, birds in the duck family have fairly normal size necks. Birds in the goose family, such as swans, have an extra large neck. It’s longer.”
Frank: “Oh.”