When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.
James Whitcomb Riley, as quoted in Exploring Indiana Highways: Trip Trivia (2007), by Michael Heim.
If it looks like a duck, and quacks like a duck, we have at least to consider the possibility that we have a small aquatic bird of the family Anatidae on our hands.
Douglas Adams, Dirk Gently’s Holistic Detective Agency (1987).