Detective Hayes entered a dark warehouse where a watchman lay tied to a chair. The watchman gasped, 'Two masked men jumped me from behind, knocked me out, and stole the shipment of microchips!' Hayes shined his flashlight on the watchman's glasses, which were smudged with grease exclusively on the outer lenses, and shook his head. What exposed the watchman's lie?
If he was knocked out from behind, his glasses wouldn't have grease on the outer lenses.
If someone is attacked from behind and knocked unconscious, their glasses would either be knocked off or the inner lenses would show smudges from pressing against the face. External smudges imply he adjusted them himself after the staged scuffle.
About this riddle
Detective riddles are tiny mysteries. Look for the detail in the story that does not fit; it is usually the clue that solves the case.
Difficulty: Medium. More like this: Detective riddles with answers or try Truth & Lie Island for a different kind of puzzle.
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