Idioms: a group of words established by usage as having a meaning not deducible from those of the individual words.
Today's idiom: "It's not rocket science".
Meaning: It is not complicated= it's easy.
Origin: After the second world war, German scientists were sent to the USA to have a leading role in "The Space Race". Their work ("rocket science") was regarded as requiring an above average level of intelligence. In the beginning, the idiom was often used in regards to football.
Quote from The daily intelligencer <a href="tel:1985">(1985</a>.): "Coaching football is not rocket science and it's not brain surgery. It's a game, nothing more."
How common is it?: highly common.
You wrote a good discussion and it is interesting to learn of similar idioms used in other languages, but...
Well, as Truman said, idioms cannot be interpreted literally, they are figurative speech and they should really only be used by native speakers or at least a learner in the upper 50 percent quartile of the advanced second language level.
There is no harm in learning their meanings which allows the learner to understand them when they're written or spoken, however, we should caution learners against using them.
Using the idioms includes the risk of using the idiom incorrectly, at the wrong time, or in the wrong circumstances, and this could result in serious embarrassment for the speaker and at a minimum, confusion for the listener or reader.
I do commend you for posting your well-written, well-explained discussion, so I gave an upvote.
Thank you for your warning, and I will stop making these.
I just wanted to share and help people learn.
My student once used "easy peasy lemon squeezy" and he was very fluent and it helped him express something he was struggling to say.
And I liked it when people commented their languages' version.
Again, thank you.
Oh, no. Not this again. I'm sure you mean well, but this has been debated to death on here. Learners should exercise extreme caution when using idioms. They usually make learners sound silly because they're misused.
It's far better to stick to standard English. Idioms aren't at all necessary for speaking or writing English. I'm sure that as a native speaker I go days with out ever using an idiom.
They're good to know in case they are encountered. But I can't recommend undertaking an active study of idioms with the goal of peppering one's speech with them.