At midnight, a jewelry store alarm blared. Police arrived to find the front glass smashed inward and display cases empty. The night watchman was tied to a chair in the back, claiming a masked man ambushed him at 8 PM, knocked him out, and tied him up. When the detective checked the watchman's wristwatch, it was ticking normally, but the hands were frozen at 3:15. What gave away the watchman's lie?
A mechanical watch would keep running unless broken, but if he was tied up at 8 PM, the watch wouldn't read 3:15 unless it ran out of power or was manipulated.
If the watch was a wind-up mechanical watch, it would keep running after 8 PM until its mainspring wound down, or if it was quartz, it wouldn't magically stop at 3:15 unless manually set or broken. More simply: a conscious man tied to a chair can still wind his watch, but an unconscious man cannot have his watch stop at a random afternoon time unless tampered with.
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: Hard. More like this: Detective riddles with answers or try Truth & Lie Island for a different kind of puzzle.
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