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" is 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”
As I noted in my answer to the related question What is a saying for "a bookish inexperience preaching the experienced", a search of Early English Books Online finds an instance of the expression in John Hawkins, The English School-master Compleated (1692) in the form "Teach your Grandam to suck Eggs."
The Wikipedia page on "Teaching grandmother to suck eggs" (cited in a comment beneath the original post above in a comment by Matt E. Эллен and in Hackworth's answer) incorrectly suggests that the expression first occurred in English in a book published fifteen years after Hawkins's collection of English proverbs appeared:
The origins of the phrase are not clear. The OED and others 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".
So Wikipedia (basing its coverage on the OED "and others") suggests that the expression may have originated in a 1707 English translation of a Spanish text—but this hypothesis is refuted by the fact that John Hawkins included an almost identical version of the saying in a list of "English Proverbs Alphabetically placed" in a book published in 1692. And if it qualified as a proverb in 1692, it had evidently been in English folk use appreciably longer than that.
The Wikipedia article goes on to offer this analysis of the expression:
Most likely the meaning of the idiom derives from the fact that before the advent of modern dentistry (and modern dental prostheses) many elderly people (grandparents) had very bad teeth, or no teeth, so that the simplest way for them to eat protein was to poke a pinhole in the shell of a raw egg and suck out the contents; therefore, a grandmother was usually already a practiced expert on sucking eggs and didn't need anyone to show her how to do it.
This description is technically inaccurate: although it might seem that the correct way to prepare an raw egg for direct-from-the-shell consumption is simply "to poke a pinhole in the shell," the proper method of preparation (as any grandmother in 1692 would have known) is to poke two holes in the egg, one at each end. This is clear from the following humorous vignette included in The Book of Anecdotes, and Budget of Fun: Containing a Collection of Over One Thousand of the Most Laughable Sayings and Jokes of Celebrated Wits and Humorists (1859):
Teach Your Grandmother to Suck an Egg
"You see, grandma, we perforate an aperture in the apex, and a corresponding aperture in the base ; and by applying the egg to the lips, and forcibly inhaling the breath, the shell is entirely discharged of its contents."
"Bless my soul," cried the old lady, "what wonderful improvements they do make! Now in my young days we just made a hole in each end and sucked."
A person who followed Wikipedia's method and poked a single pinhole into an egg would probably find the subsequent sucking operation far less satisfactory than anticipated. That isn't to say that removing the contents of an egg without breaking two holes in the shell is impossible. Weasels seem to manage quite well after creating a single aperture—and their skill at the task was well known in Shakespeare's time, as we see in this exchange from As You Like It (1599):
Jaques. More, more [song], I prithee, more.
Amiens. It will make you melancholy, Monsieur Jaques.
Jaques. I thank it. More! I prithee, more. I can suck melancholy out of a song as a weasel sucks eggs. More! I prithee, more.
But according to Mark Mancini, "How Did Weasels Get Such a Bad Rap?" (June 4, 2019), weasels don't actually suck an egg's contents out of the shell:
The short-tailed weasel (Mustela erminea) will put a small hole into an egg and then lap up the contents as they come oozing out. What they don't do is actively suck out the yolks and the whites, like some kind of breakfast food vampire.
Sucking an egg from a shell that has a single hole in it is likely to be no more efficient than "shooting" a beer from a can that has only one opening in it. Consequently, it seems to me (for the same reasons that 1006a gives in a comment beneath Callithumpian's answer) that "sucking an egg" does not fall into the category of things (like "falling off a log") that require no art at all. Nevertheless, the technique for doing it properly is easy to learn and requires no great skill, so it isn't something that a person of advanced years living in England in the late 1600s would not have known.
As for the Wikipedia article's surmise that sucking raw eggs was a practice limited to the old and toothless, that notion seems contrary to the historical record, too. Consider this excerpt from James Adair, The History of the American Indians; Particularly Those Nations Adjoining to the Mississippi, East and West Florida, Georgia, South and North Carolina, and Virginia (1775):
None of the Indians however eat any kind of raw sallads ; they reckon such food is only fit for brutes. Their taste is so very opposite to that of cannibals, that in order to destroy the blood, (which with them is abomination to eat) they over-dress every kind of animal food they use. I have often jested with them for pressing me to eat eggs, that were boiled so much as to be blue, and told them that my teeth were too bad to chew bullets. They said they could not suck eggs after the manner of the white people, otherwise they would have brought them raw ; but they hoped I would excuse the present, and they would take particular care not to repeat the error, the next time I favoured them with a visit.
Evidently, the reason that the Indians "could not suck eggs" wasn't because they didn't know the trick of opening a raw egg at both ends; rather, they "could not" because eating the egg uncooked was abominable to them. But their awareness of "the manner of the white people" in sucking raw eggs suggests that this practice was fairly widespread among white people (in North America, anyway) at the time. I do not interpret Adair's joke that his teeth weren't strong enough to chew bullets as an indication that he and other white people characteristically sucked raw eggs because they had no (good) teeth. In any case, the Indians don't impute the practice to "old white people" or to "toothless white people" but to "the white people."
"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'"