The Moon is made of green cheese

"The Moon is made of green cheese" is a statement referring to a fanciful belief that the Moon is composed of cheese. In its original formulation as a proverb and metaphor for credulity with roots in fable, this refers to the perception of a simpleton who sees a reflection of the Moon in water and mistakes it for a round cheese wheel. It is widespread as a folkloric motif among many of the world's cultures, and the notion has also found its way into children's folklore and modern popular culture.

The phrase "green cheese" in the common version of this proverb (sometimes "cream cheese" is used), may refer to a young, unripe cheese or to cheese with a greenish tint.

There was never an actual historical popular belief that the Moon is made of green cheese (cf. Flat Earth and the myth of the flat Earth). It was typically used as an example of extreme credulity, a meaning that was clear and commonly understood as early as 1638.