In my native language, we use an idiom to warn someone that they're doing something which has no result at the end:
Trying to convince him is like squashing water ...
Is there any idiom in English with the same meaning?
There are a few idioms like that. One is:
You're just beating your head against the wall.
Like trying to squeeze blood from a turnip.
and another is:
It's like trying to herd cats.
They each are used is slightly different situations. To be most analogous to your expression it sounds like I'd use the first one: Trying to convince him is like beating your head against the wall
Sisyphean labor. "Sisyphean" means endless and unavailing, as labor or a task. Comes from Greek mythology, Sisyphus was made to roll a giant boulder up a mountain as punishment by the gods, but the boulder would always roll down before he could get it to the top, forcing him to begin the pointless exercise anew.
There are very many idioms that you could use for a Sisyphean task. Take your pick.
I think the most generally applicable would be "pushing water uphill with a rake".
If you want to say that you're doing the right thing, but you're doing it too late to have any effect, you'd say "shutting the stable door after the horse has bolted". This is sometimes known as "stable door shutting" for short.
If you want to say that there are too many unpredictable factors to make your task possible, you'd talk about "herding cats".
If you want to say that you're trying unsuccessfully to get someone to do something or give you something, this is "trying to get blood out of a stone".
Plus all the other excellent suggestions in the other answers here.
It's a fool's errand
To add to the other good examples:
Rearranging the deck chairs on the Titanic.
This is my goto for these situations because I think it really creates a nice visual of futility.
I would use ... like beating (or flogging) a dead horse
It even has its own wikipedia entry
I would not use Jim's suggestion beating my head against the wall with the word like - so
I am banging my head against the wall trying to convince him
Trying to convince him is like flogging a dead horse.
are two ways of expressing your problem
If the original is Water naar de zee dragen then it is a Sisyphean task
Like trying to carry water in a sieve
Like trying to empty the sea with a thimble
I did a search for "is like trying to convince a" and came up with a lot of really amusing results:
like trying to convince a cat it wants to go swimming.
like trying to convince a cat anything.
like trying to convince a two-year-old that a shot won't hurt.
like trying to convince a five-year-old that vegetables taste like candy.
like trying to convince a pacifist that violence solves everything.
like trying to convince people that "white trash" is derogatory and "redneck" isn't.
like trying to convince a mouse that the cat is her best friend.
like trying to convince a corpse that physical therapy is the only answer to its problem.
like trying to convince a newborn that crying is illogical.
like trying to convince a scientist that the moon is really made of green cheese.
Depending on context, Pissing in the wind.
trying to push on a rope
to express doing something in a way that is clearly not going to produce the desired results.
English is not my native language
However, some time ago I read something like taking coals to Newcastle.
In the specific case that you are trying to convince to someone who will not listen, you are talking to a brick wall or a blank wall.
Something that fits and has not yet been mentioned:
It's like swimming in treacle*
*Substitute treacle for your local high-viscosity liquid.
is what I use. As in a car that's stuck in the mud or a bank of snow.
Usually used in the context of assigning useless busy work to keep employees busy when there's no real work to do.
You're on a hiding to nothing trying to convince him.
It means that your possible outcomes range from heavy defeat (a hiding) to nothing, and they exclude even a minor victory. It's not quite as evocative as other idioms, but it's commonly used.
An example would be "a wild goose chase". An example of the use of this is:
I tried to tell him he was going on a wild-goose chase.
The Online English Wiktionary states that:
Early recorded use refers to a type of 16th century horse race where everyone had to try to follow the erratic course of the lead horse, like wild geese have to follow their leader in formation.
Shooting at the sun
Shooting at the moon
Killing a Dodo
Racing a Cheetah
Barking up the wrong tree
Wikipedia: if you are barking up the wrong tree, it means that you have completely misunderstood something or are totally wrong.
There is a very similar idiom in English, treading water, but it doesn't exactly fit into the sentence you've provided. It means working very hard yet achieving nothing.
We've tried to fix it, but it's as if we're treading water.
Man I tell you what, I felt like A one legged cat try'in to burry turds on A frozen pond out there.
-Tom Anderson from Beavis and Butthead
There is a plethora of expressions you could use for this, but I've compiled a list of phrases that I've heard which I don't think anyone else has put in their answers. Here are a few of them...
How about "He might as well be pissing on a forest fire" ? Totally futile.
"Tilting at windmills" a reference to Don Quixote who imagined the windmills to be giants attacking a village.
Here are two from Sting's "Be Still My Beating Heart" (from ...Nothing Like the Sun ):
"Never to be wrong,
Never to make promises to break.
It's like singing in the wind.
Or writing on the surface of a lake."
I came here to add "Pissing in the wind," as that's quite common, but already mentioned. However, I haven't yet seen "Pissing up a rope" mentioned. That one is sometimes used as an insult (e.g. "He can go piss up a rope"). It's possibly a corruption of "pushing a rope," also a fairly futile thing to attempt.
I think "talking to a wall" is a good option, especially if you rephrase your original slightly: "Talking to a wall would be as effective as trying to convince him of anything."
DUST IN THE WIND
If the intent of the usage is to point out the non-durability of one's efforts, which is to say that what we do will vanish in time, then the song "Dust in the Wind" by the musical group KANSAS provides (uses) phrases like:
Beat the air or beat the wind - to carry on making futile efforts. In my native language, we`ve an identical idiom in meaning. Crush/squash the water.
a lost cause
a cause with no chance of success
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