teach grandma how to suck eggs

• don't teach your grandma to suck eggs, don't teach your grandmother how to suck eggs, don't teach your grandmother to suck eggs, teach grandma to suck eggs
• don't teach your grandmother how to milk ducks
• don't teach your grandmother to steal sheep

First attested circa 18th century, from Spanish.

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teach grandma how to suck eggs (third-person singular simple present teaches grandma how to suck eggs, present participle teaching grandma how to suck eggs, simple past and past participle taught grandma how to suck eggs)

1. (idiomatic, chiefly in the negative) To tell an expert how to do things.

• don't try to teach grandma how to suck eggs