The moon is made of green cheese

Origin

The phrase "green cheese" doesn't refer to color, but to cheese that is young, fresh, and unripened—often having a pale, somewhat greenish tint. This whimsical notion of the moon as a giant dairy product can be traced back to medieval folklore, where it served as a playful metaphor for a ridiculous, impossible idea. It was a common taunt, suggesting that only the most gullible or foolish person could possibly believe such an obvious absurdity, given the moon's distant, pockmarked appearance. The phrase became a perfect, pithy way to expose a lie or a naive belief, implying that anyone who fell for it was truly out of touch with reality.