<P> 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 . </P> <P> A claimed recent application of the duck test was the denial of tax exempt "nonprofit" status to Blue Shield of California . </P> <P> The Liskov Substitution Principle in computer science is sometimes expressed as a counter-example to the duck test: </P> <P> If it looks like a duck and quacks like a duck but it needs batteries, you probably have the wrong abstraction . </P>

If it walks like a duck looks like a duck