marbles

Reader's Digest 10月号に出ていた Simple steps to good healthの一つです。
Keep your marbles. Just 1.6km a day might halve your chance of developing dementia. Amazingly, US study found that walking increased grey matter in the elderly.
"marble" の意味として大理石とビー玉しか知らなかったのですが、"dementia" とか "grey matter" があるので脳あるいはその働きに関係する意味だと分かります。　辞書を見ます。
・Merriam-Webster's Online Dictionary: plural : elements of common sense; especially : sanity: persons who are born without all their marbles - Arthur Miller
・American Heritage Dictionary: marbles (used with a sing. verb) Slang Common sense; sanity: completely lost his marbles after the stock market crash.
・Macmillan Dictionary: "lose your marbles" informal to become crazy: Have you lost your marbles? We can’t do it that way!
この様な意味は必ず複数形の"marbles" が使われるので、"marble" で辞書を引くと上記の意味を載せていない辞書も多い。　しかし、この意味の "marbles" を使ったイデオム、フレーズが下記の様に辞書に載っています。
・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs:.
1. have all one's marbles: Fig. to have all one's mental faculties; to be mentally sound. (Very often with a negative or said to convey doubt.) I don't think he has all his marbles. Do you think Bob has all his marbles?
2. lose (all) one's marbles and lose one's mind: Fig. to go crazy; to go out of one's mind. What a silly thing to say! Have you lost your marbles? Look at Sally jumping up and down and screaming. Is she losing all her marbles? I can't seem to remember anything. I think I'm losing my mind.
not have all one's marbles
3. not have all one's marbles: Fig. not to have all one's mental capacities. John acts as if he doesn't have all his marbles. I'm afraid that I don't have all my marbles all the time.
否定的な意味で使われることが多いとありますが、RDでの例の様に肯定的で動詞も　"have" や "lose" ではなく　"keep" が使われる事もあるのですね。