Pointless effort

Question

There is a saying in Japanese "kappa ni suiren" which translates as "swimming lessons for a kappa". (A "kappa" is a mythical river imp / water dwelling monster). The only similar expressions that I can think of in English are: "Teach your grandmother to suck eggs!" (which I haven't heard anyone use in a long time) and "Carry coal(s) to Newcastle". I'm looking for an "refran" in Spanish that conveys this sense. The sort of thing that can be said to someone who has just tried to "teach/explain" something that you were already well aware of; along the lines of "Tell me something that I don't know!" (but a "refran" not just a translation).

Answer 1

In Spanish, I've heard the saying, "Descubrió el agua tibia," as in he/she thinks he/she discovered something grand even though it's more like common knowledge. There's another that says, "enseñándole a su mamá a hacer hijos," but I've only ever heard my mother tell me this so I don't know if it's appropriate to say to anyone else.

Answer 2

James Santiago said:

It was first recorded in 1707 in a translation by John Stevens of the collected comedies of the Spanish playwright Quevedo: 'You would have me teach my Grandame to suck Eggs'.

Fascinating! I couldn't find the original Spanish anywhere, but that would certainly answer samdie's question.

Indeed it would! Of course if it were to turn out that the reason I learned the expression in English was because of reading something by Quevedo, I'd be very embarrassed.

Answer 3

LadyDi said:

In Spanish, I've heard the saying, "Descubrió el agua tibia," as in he/she thinks he/she discovered something grand even though it's more like common knowledge. There's another that says, "enseñándole a su mamá a hacer hijos," but I've only ever heard my mother tell me this so I don't know if it's appropriate to say to anyone else.

"enseñándole a su mamá a hacer hijos" Now, that one, I really like!