A: I’ve promised to treat everyone in my class to dinner tomorrow because it’s my birthday.

    B: Where?

    A: The newly opened Western restaurant on Waterfront Avenue.

    B: You must have lost your marbles. That restaurant is very expensive.

    Note: The idiom means to “lose one’s mind.” Marbles are the little glass or metal balls that children use to play the eponymous game. From the mid-19th century “marbles” was also used to mean “personal effects,” “goods” or more generally “stuff.” The notion of “losing something that is important to you” appears to have migrated from the image of a forlorn child having lost his prized playthings. An early citation of this figurative usage is found in an August 1886 copy of the St. Louis Globe-Democrat: He has roamed the block all morning like a boy who had lost his marbles.