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Chaff is the seed casings and other inedible plant matter harvested with cereal grains such as wheat. The chaff must be separated from the grain before use, by such techniques as threshing and wind winnowing. The word "Chaff" is also used to refer to something worthless, such as in the expression "separating the wheat from the chaff", meaning to find things of value and separate them from things of no value.
Chaff is also an anti-radar technique in which aircraft or other targets spread a cloud of small, thin bits of aluminum or plastic, which appears as a cluster of secondary targets on radar screens. Chaff was first used by bombers during World War II as a defense against anti-aircraft artillery. Modern armed forces use chaff to distract radar-guided missiles from their targets.