Politicians lie. Not all of them. Not every time. But most of them, from both "major" political parties, lie. A lot. It's not always easy to tell when they're lying. It's not always easy to prove they're lying. Often, it's not even easy to tell if they're just lying to us or to themselves as well. Some politicians want Facebook to stop politicians from lying. They phrase that desire as a request for Facebook to "fact-check" content posted by politicians, especially political advertising.