marbles
n. plural-onlyn. small, round balls made of glass or stone that children use to play games. In a casual way, it can also mean someone's ability to think clearly or stay sane.
n. small, spherical toys typically made of glass, clay, or stone, used in various traditional games. In idiomatic usage, refers to one's mental faculties or sanity.
The children are playing with colorful glass marbles in the park.
After working sixteen hours a day for a month, he felt like he was losing his marbles.
While the physical game of marbles has declined in popularity, the idiomatic expression regarding one's cognitive stability remains a staple of colloquial English.
When referring to the game or the objects, it is a plural countable noun; in the idiomatic sense 'lose one's marbles', it is always plural.
He lost his marble.He lost his marbles.The idiom for losing one's sanity always uses the plural form 'marbles'.