Now you can mark answers as being the Solution to a certain topic.
Click the ellipsis … at the bottom of the post:
And then the checkbox 
Solution there:
These posts will then be highlighted on top of the topic so that newcomers can read the solution immediately:
Sounds amazing! Do we want to look at marking some of the already solved ones in this way? Also does this let us search for solved ones?
I don’t see an issue with marking past topics as solved, if you happen to pass them by. But I’m certainly not going to go over all of the past topics 
About Searching:
When searching, you can put
in:solvedorin:unsolvedto only show solved/unsolved topics.
You can also append
?solved=yesto a topic list URL to filter the view. For example,https://meta.discourse.org/latest?solved=yeswill filter the homepage to only show filtered topics.https://meta.discourse.org/c/support?solved=yeswill show solved topics in the #support category.
There are talks about adding this to the UI more prominently, but it doesn’t exist currently.
Great!
Is it possible to mark more than one answer as solution (sometimes the solution is reached gradually and incrementally)?
Is it possible to mark more than one answer as solution?
No, I just tried it, and when you select a second post as “solution” it clears the mark from the previous post.
So you can update the post that is the solution to a newer one, but you can’t have more than one at once.
Maybe the solution is to create a reply to compile the solution in one answer or add quotes to, again, have only one answer
Yes, I agree this sort of thing can work well.
Another (different) thing that we can use sometimes is topics that are “wikis”, meaning anybody can edit them. In this case it is not a matter of marking solutions, but simply of keeping the initial post that starts the topic as a complete reference of all the information gathered so far. This is great for tutorials.
I agree too on AlxGr’s suggestion.
By the way, I tried cliccking on the three dots ellipsis but, after clicking, I only get a flag and a bookmark sign. I don’t get the solution checkbox nor the bin and the wrench tool.
Maybe that is the way the menu looks for admins… but the “solution” check box is meant to be usable by everyone.
Were you logged in?
Exactly which post did you try this on?
Yes I was logged in and I tried it on this topic (I clicked the ellipsis on all the posts of the topic) with no success.
Maybe there is somewhere an option to allow normal users to do it in addition to admins.
I think any user can “mark” as solution, but only on their own posts. Moderators can mark on anybody’s post, I believe.
By the way, congratulations, you just became a moderator. 
Thanks Pedro!
PS: I tried to write just “Thanks Pedro!” and a popup appeared saying: “Post must be at least 20 characters”, so I had to add the PS.
