A crude, but often effective, rule for eliminating unlikely possibilities and identifying a most likely or most correct assessment.

Taken from the saying, "If it walks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck."

----

"Swims like a duck" is less frequently heard than "looks like a duck," but it hardly matters.

Interesting perversions:

	 :	''"If it looks like a duck, quacks like a duck and walks like a duck - then it is probably a duck."'' �- Old saying

	 :	''"If it looks like a duck, walks like a duck, and quacks like a duck, it's probably a duck."'' -- SenatorJosephMcCarthy, in a 1952 speech, suggesting a method for identifying communists and communist sympathizers.  http://www.gantthead.com/article.cfm?ID=86476

	 :	''"But, what kind of duck is it?   What are the ideological underpinnings of the duck?  What are the operational imperatives of the duck?  Is the duck a cat in disguise? Does the duck/cat have allies?  What are the rules of engagement for duck/cats?  Do duck feathers change? - Operation Other Than War thoughts on a conventional concept�.."'' -- Dean Rose, "Operations Other Than War: A Modeling and Simulation Imperative" http://ms.ie.org/SIW_LOG/98F/98F-SIW-007.doc

	 :	''"If it walks like a duck, quacks like a duck and looks like a duck, we professionals say it's a duck....  They want a smoking duck."'' -- Unnamed intelligence analyst quoted in Washington Post, 2003-03-16.

	 :	''"The answer, of course, depends on whether or not the name `duck' is a trademark! If it is, then the closest that something can get, without permission of the owner of the trademark, is `duck-like.'"'' -- http://www.netbsd.org/Misc/call-it-a-duck.html

	 :	''"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."'' -- DouglasAdams

	 :	''"It looks like a duck, it walks like a duck, it quacks like a duck, but what the hell is it?"  ("Overlooking the sense of the obvious.")'' -- http://www.ta-tutor.com/WEBPDF/ducknewc.pdf

	 :	''"If it looks like a duck, and walks like a duck, and quacks like a duck, it's a toxic waste incinerator."'' -- Jane Williams, chair of the Sierra Club's waste committee, commenting on the tendency of nixed waste management proposals to keep coming back under different names.

----

"Smoking duck" is a reference to the metaphorical "smoking gun."  However Smoking Duck is also a breed of duck.

"What's the matter... Ain't you ever seen a smokin' duck before?" -- Daffy Duck, after getting blasted by Elmer Fudd?

----

There is an interesting TV adaptation of this saying in a Dexter's Laboratory episode ("Ultrajerk 2000").
Dexter, the boy genius, builds a small sentient robot as a lab assistant to improve his home research facilities, then he takes a day off.
In the meanwhile, the robot builds laser-equipped fixed terminals and upgrades himself into a huge tower.
When Dexter comes back, the robot decides to terminate him as "inefficient" and his lasers start shooting at him. Dexter hides away and his sister, completely unaware of the tense situation, comes into the lab. She starts talking with the tower-robot as if it was Dexter in disguise: it looks (!), speaks and behaves like Dexter, so it must be her brother. The laser terminals, convinced by her words, dutifully destroy the tower and, without guidance, go offline themselves.

LorenzoGatti
----
If it walks like a duck.. swims, quacks like one... it might not be a duck:

http://www.whatbird.com/browse/objs/all/birds_na_147/20/body%20shape/303/duck-like

----
See also DuckTyping
----
CategoryIdiom