Three doctors are in the duck blind and a bird flies overhead.
The general practitioner looks at it and says, "Looks like a duck, flies like a duck... it's probably a duck," and shoots at it but misses and the bird flies away.
The next bird flies overhead and the pathologist looks at it, then looks through the pages of a bird manual, and says, "Hmmmm... green wings, yellow bill, quacking sound... might be a duck." He raises his gun to shoot it, but the bird is long gone.
A third bird flies over. The surgeon raises his gun and shoots almost without looking, brings the bird down, and turns to the pathologist and says, "Go see if that was a duck."