这则成语出自《庄子·秋水》。井底的青蛙认为天只有井口那么大，比喻那些见识短浅的人。

This idiom comes from "Zhuangzi: Autumn Water" (3rd century BC). The frog at the bottom of the well thinks that the sky is only as big as the mouth of the well, which is a metaphor for those who are short-sighted.