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, 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.