teach (one's) grandmother to suck eggs

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teach (one's) grandmother to suck eggs

To try to teach an older person who is wiser and more experienced and wordly than a young person may think. Why are you explaining basic typing to Ethel? Yes, she's 70, but she's been using a computer since before you were born—quit teaching your grandmother to suck eggs.
See also: egg, grandmother, suck, teach