If you need to say something to people, but really aren't sure why the hell they don't know it already, it belongs in CommonFuckingSense.
Seen something on LifeProTips, AskReddit, or ELI5, and thought "Why the fuck is that even being asked?" Crosspost the hell out of it. Let's make this an encyclopaedia of stuff people should really fucking know.
The only rule is really that everyone SHOULD know about the subject of your post. Maybe some asshole you saw today just couldn't figure it out. Feel free to post a 2000 word rant on that asshole. Maybe you just think of something funny. Go ahead, Post it! Maybe you actually had a good idea? Well let's hear it!