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?