I used an expression a few days ago, that something was like teaching one’s grandmother how to such eggs. The expression means that the advice being given is so basic, or that the subject is already so well known to the person being spoken to, that they must already know it quite well. There is also the meaning that someone young should not offer basic advice to someone who is older and more experienced.
It touched off a discussion with my wife and son as to whether grandmothers suck eggs, how the expression originated, and a request for a Bytes on the subject.
The Oxford English Dictionary suggest that it comes from a translation in 1707, by J. Stevens, of Francisco de Quevedo (Spanish author):
"You would have me teach my Grandame to suck Eggs."
Henry Fielding used the expression in his 1749 book “The History of Tom Jones, a Foundling”:
“I remember my old schoolmaster, who was a prodigious great scholar, used often to say, Polly Matete cry Town is my daskalon. The English of which, he told us, was, That a child may sometimes teach his grandmother to suck eggs”
Percy Bysshe Shelley, in a letter to Leigh Hunt, dated 15 August 1819, wrote:
“But what am I about? If my grandmother sucks eggs, was it I who taught her?”
As to why “suck eggs”, something grandmothers don'tr do any more, here is an explanation by a contributor to Wordwizard:
Perhaps its meaning is getting lost in time as few people nowadays literally suck eggs. Many years ago people would suck out the egg contents by piercing the egg at both ends and then sucking on one of the ends. You could reverse the procedure and blow out the contents also. It was such a commonplace procedure then that to "teach your grandmother to suck eggs" was like a child trying to teach as new something the grandmother well knew how to do. The saying still survives despite the fine art dying out in our "civilized" and salmonella fearing culture.