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!" He 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, "Hmmm... 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 and brings the bird down. He turns to the pathologist and says, "Go see if that was a duck."