teach your grandmother to suck eggs
teach (one's) grandmother to suck eggs
To try to teach an older person who is wiser and more experienced and worldly 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.
teach your grandmother to suck eggsBRITISH
If you teach your grandmother to suck eggs, you give advice about a subject to someone who knows more about it than you do. Look, I don't want to teach my grandmother to suck eggs, but haven't you done this the wrong way round? Note: You can also say that you teach your granny to suck eggs. At the risk of teaching my granny to suck eggs, wouldn't it be better to use this pan?
teach your grandmother to suck eggspresume to advise a more experienced person.
The proverb you can't teach your grandmother to suck eggs has been used since the early 18th century as a caution against any attempt by the ignorant or inexperienced to instruct someone wiser or more knowledgeable.