This article is from the Puzzles FAQ, by Chris Cole email@example.com and Matthew Daly firstname.lastname@example.org with numerous contributions by others.
When someone posts a puzzle that you know the solution to, or better yet,
a puzzle that you discover the solution to after hours of thinking, it's
natural to want to post the solution to proclaim your achievement to the
world. Such posts are called "spoilers". There's nothing wrong with
posting a spoiler, but certain guidelines should be followed.
When you post a spoiler, you should include the warning [SPOILER] after
you quote the puzzle, but before you include your solution. (Some people
also include [SPOILER] in the subject line of their post, although
changing subject lines makes it difficult for some to follow the thread of
a discussion, so it is frowned upon.) After the [SPOILER] flag, insert a
page break by pressing Ctrl-L; it should look like this: ^L. The page
break will cause many newsreaders to pause at that point and give the
reader a chance to move on to the next post if they'd rather wait before
reading your spoiler. If your editor doesn't allow pagebreaks, 24 or more
blank lines will do.