teach one's grandmother to suck eggs

teach one's grandmother to suck eggs (to try to tell or show someone more knowledgeable or experienced than oneself how to do something; to give advice to someone about a subject that they already know more about than you )

Example: Don't teach your grandmother to suck eggs. Bob has been playing tennis for years. (McGraw-Hill Dictionary of American Idioms and Phrasal Verbs)
Example: Don't suggest showing Mary how to knit. It will be like teaching your grandmother to suck eggs. (McGraw-Hill Dictionary of American Idioms and Phrasal Verbs)

Example: You're teaching your grandmother to suck eggs, Ted. I've been playing this game since before you were born! (CID)

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