Lost his marbles?

[dina]

Test No. incompl/inter-62 "Losing It (2)", question 2

There's no hope for him now because he's lost his ......... and has gone completely crazy.

(a) glasses
(b) stones
(c) jewels
(d) marbles

Test No. incompl/inter-62 "Losing It (2)", answer 2

There's no hope for him now because he's lost his marbles and has gone completely crazy.

Correct answer: (d) marbles

Your answer was: incorrect
There's no hope for him now because he's lost his jewels and has gone completely crazy.
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There's no hope for him now because he's lost his marbles and has gone completely crazy.

whats the meaning of lost his marbles, why marbles?
thanks

[Torsten]

If you lose your marbles you go crazy, you lose your mind.
It's an idiom.
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