teach grandmother to suck eggs
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teach (one's) grandmother (how) to suck eggs
To try to teach an older person who is wiser, more experienced, and more worldly than a young person may think. Why are you explaining basic typing to Ethel? Sure, she's 70, but she's been using a computer since before you were born—quit teaching your grandmother to suck eggs! These young hotshots come in and try to prove they know the ropes better than we do, like they're teaching their grandmothers how to suck eggs.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
teach one's grandmother to suck eggs
Fig. to try to tell or show someone more knowledgeable or experienced than oneself how to do something. Don't suggest showing Mary how to knit. It will be like teaching your grandmother to suck eggs. Don't teach your grandmother to suck eggs. Bob has been playing tennis for years.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.