I’ll teach your grandmother to suck eggs.
To give advice to a person on a subject about which they know more than you.
First seen in an early eighteenth century English translation of a play by the Spanish author Francisco de Quevedo, the phrase has since been used by such lauded English authors as Henry Fielding and Percy Shelley, as well as in the classic Ren & Stimpy tune, “Happy Happy, Joy Joy.”
So after I taught the new girl how to log on to the server, it turns out she’s I.T. She didn’t say anything, basically just sat there for 10 minutes letting me teach my grandmother to suck eggs.