I like asking questions on this site. Can I pick some interesting puzzle/riddle from the internet and ask it here?
You can. That doesn't necessarily mean you should.
There's more to Puzzling.SE than puzzles posted as challenges for others to try to solve. While it's definitely true that these puzzle challenges are the largest share of the questions posted here, that's not because the site is all about posting puzzles for others to solve. Puzzling Stack Exchange is a question and answer site for those who create, solve, and study puzzles. Challenge questions allow those users who enjoy creating puzzles and/or want to improve their skill at it to post their own creations for others to solve and offer feedback on; they can often give other creators new ideas for how puzzles can be themed, designed, organized and presented; and they give those users who enjoy solving puzzles new challenges on which to test themselves.
So while there are a lot of puzzles posted here, the aim of the site is not to collect every puzzle and riddle into one place. It is more about sharing either original puzzles, or ones that are very interesting that most people probably wouldn't have seen, with a community of puzzle fans. Please don't try to make this site a global collection of every puzzle, or even every "good" puzzle, that has ever been made; it's an impossible task, and not one we want done anyway.
Note, too, that we generally don't accept questions from ongoing tests, competitions or contests, and we definitely don't accept plagiarised content. For content that you did not create yourself, you always need to provide proper attribution - at minimum you need to let us know where it came from, including the name of the author and/or site of origin and a link to the original content. Posts which use someone else's content without disclosing where it came from are generally deleted.
If done correctly, sharing puzzles you find elsewhere can be perfectly fine; indeed, sharing really memorable puzzles found elsewhere is appreciated. But it's not really fair (nor fair use) for someone to gain reputation on this site by predominantly (or exclusively!) posting run-of-the-mill content from other sources, even with attribution. Reposting numerous generic puzzles from other Internet sites can be a lot like someone posting a Sudoku book, one puzzle every few days... that shouldn't be okay here, for multiple reasons.
- Be selective. We vastly prefer quality over quantity.
- Don't just do reposts. It looks like rep farming, begging the question of why you're doing it.
- Always provide proper attribution. Don't plagiarize!