Here are the rules: report an idiom only if you, yourself, have actually used it with the last few weeks, or you have heard a native speaker use it in person (in real life, not on TV or in a movie).
I'll start things off with two of them.
1) <em>To sneak a peek</em>: to look quickly at something; often, to look at something secretly and hope nobody notices.
"I didn't want to pay for the magazine, so I couldn't really read the article carefully, but I was able to sneak a peek at it."
"She'd left the checkbook open, right out on her desk, and I snuck a peek at it. I couldn't believe the balance!"
2) <em>That's water over the dam--</em>something was a mistake, but it's in the past and nothing can be done about it, so let's not worry about it or complain about it.
"I told you not to stock up on so many ink cartridges, now the printer's broken and they won't work in the new printer, and they can't be returned."
"Yes, you were right. Sorry."
"Oh well, it's water over the dam."
It's funny how language evolves. It's so gradual that you don't realize things have changed until the change is so old that you can't remember when it happened.
It's usually difficult to identify any source or explanation of the change. Sometimes you'll know--it will be some celebrity's catchphrase, or it may have been used in a politician's speech--but that's the exception and not the rule.
The three word phrase "long story short" is interesting, because the idiom "to make a long story short" has been around forever. Excuse me a second while I make a Google search. I found it in an 1819 collection of the essays of Oliver Goldsmith:
"To make a long story short, he wanted a servant, and hired me."
But it might be much much older than that.
However, shortening it to the three words "long story short" is recent. It's happened in my lifetime. And of course I can't remember exactly when. It's not the sort of dramatic event that creates a memory. I am thinking I first heard it in the 1980s or 1990s.
All good ones. In the UK, we say "water under the bridge" instead of "water over the dam". All the others mentioned so far are the same in the UK.
Here's one : "to be up the creek".
This means : "to be in a difficult situation without any obvious solution". As "creeks" are more common in the USA than the UK, I'll wager that "up the creek" is common in the USA too, but I can't say for sure.
Yes, "to get into hot water" is frequently used, and the idiom itself is completely inoffensive. Parents say it to kids--"You'll be in hot water if you don't have your room picked up before Mom gets back."
"I hope you'll change your rules and allow some television or internet quotes as well." OK, as long as it is live TV, such as a talk show or news interview... or from someone posting casually on the Internet. The point is that it needs to be an English speaker using English naturally, without thinking. It can't be something a scriptwriter wrote, scriptwriters love words and language in sitcoms or dramas or movies is always much more interesting, entertaining, and higher in intensity than language in real life.
I've heard and spoken the phrase "a baker's dozen" in real life.
Considering that many of us are not around English speakers in real life, I hope you'll change your rules and allow some television or internet quotes as well ;-)
Last week, after I heard someone use the phrase on tv, I searched the internet for the story behind "a baker's dozen".
I've known the meaning for a long time, but I was curious why a baker's dozen would be thirteen. Interesting theories!