You cannot teach a crab to walk straight.

The saying uses a crab’s sideways gait as a metaphor for ingrained nature or habit: some dispositions are so fixed that instruction cannot “straighten” them. In this sense it functions like a proverb about the limits of education and reform—especially when confronting someone whose character, training, or incentives push them persistently in the wrong direction. Attributed to Aristophanes, it fits the comic poet’s frequent use of earthy animal imagery and proverbial wisdom to puncture pretension and moralize through humor. However, without a secure textual location in Aristophanes’ surviving plays, it should be treated as a later proverb or paraphrase associated with him rather than a firmly documented line.