This is such a strange idiom, all I could find with a Google search was the meaning of it, but not where it came from. When you're telling somebody something they already know well, it's sometimes said that you're "teaching grandma to suck eggs". Where did this phrase come from, and what is 'suck eggs' referring to?
The Phrase Finder has "Don't try to teach your Grandma to suck eggs" as older than you might think, but without any explanation of the egg sucking part.
Don't offer advice to someone who has more experience than oneself.
These days this proverbial saying has little impact as few people have any direct experience of sucking eggs - grandmothers included. It is quite an old phrase and is included in John Stevens' translation of Quevedo's Comical Works, 1707:
"You would have me teach my Grandame to suck Eggs."
A little more on the egg-sucking part from 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.
Phrase finder quotes a 1542 "to teach our dame to spyne" as a possible origin. The spinning version makes perfect sense: even as late as the 1500s, all women learned how to spin, and many of them spent most of their waking hours with a spindle in their hands.
At some point, however, the logical metaphor lost favor, and the phrase mutated: instead of teaching your grandma something she knows better than you because she's been doing it since she was three years old, the phrase became about things that anyone with a modicum of intelligence can figure out how to do, if only they could come up with a reason to do them in the first place.
I think it is incorrect to try to assign a literal meaning to the "suck eggs" part, like that thread on Wordwizard tries to do. The simile is meant to be absurd. In fact, I see this as a relative of phrases such as "ass over teakettle" or "it's not rocket surgery": we know what they mean because we know the original phrases they refer to, not because they make any sense.
The earliest dictionary definition I found is from Dictionarium Britannicum: Or, A More Compleat Universal Etymological English Dictionary Than Extant (1736) by Nathan Bailey, George Gordon, Philip Miller, Thomas Lediard:
Teach your Grannum (Grandame) to suck Eggs (A Reproof to those, who think they have more Knowledge than the whole World, and will be ever and anon teaching those who have had more Experience than themselves.
The Scots say : Learn your Goodam to make Milk-Kail (Milk-Pottage) or, Teach your Father to get Bairns (Children) Lat. Sus Minervam [pig teaching Minerva, the goddess of wisdom], Fr. Les Oisons mènent les Oyes paître (i.e.) the Goslings lead the Geese to the Pasture. We say likewise, teach your Granny to grope her Goose. The It. I paperi voglien menar a bene l' oche.
Apparently some people, especially older village women, had the secret to tell by touch whether a goose or duck was likely to lay eggs, and teach your grandame to grope her ducks is even older, from 1611 (more on 16th century poultry groping here). Similarly, to teach her to sup sour milk is from 1670 and an Irish version was to milk eggs.
Eric Partridge, in his Dictionary of Slang and Unconventional English, gives this reading:
grandmother ( or granny) how to (or to) suck eggs, teach one's.
To give advice to one's senior; esp. to instruct an expert in his own expertise: from ca. 1600. Cotgrave, Swift, Fielding. Occ. from ca. 1790, abbr. to teach one's grandmother or granny.
He goes on to say suck eggs may be a mutation from to spin (as in with a distaff), etc., and other parallels are to grope ducks, or to sup sour milk. Presumably the sour milk and egg-sucking give the phrase a derisive, negative quality.
Teaching grandmother to suck eggs is an English-language saying, meaning that a person is giving advice to someone else about a subject that they already know about (and probably more than the first person).
The origins of the phrase are not clear. The OED and others suggests that it comes from a translation in 1707, by J. Stevens of Quevedo (Spanish Playwright)
Its use was recorded in Tom Jones by Henry Fielding, published in 1749:
“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”
"Teaching grandmother to suck eggs is an English-language saying, meaning that a person is giving advice to someone else about a subject that they already know about (and probably more than the first person)"
"The origins of the phrase are not clear. The OED and others suggests that it comes from a translation in 1707, by J. Stevens of Quevedo (Spanish Playwright):
'You would have me teach my Grandame to suck Eggs'"