Facebook has been willing to run ads in the middle of videos, but it has historically been reluctant to put ads in front of videos and irk people just trying to watch a clip that surfaced in their News Feed. However, it might be willing to break with tradition in a limited way. The social network plans to test pre-roll ads in the Watch tab and other spaces where you "intentionally go to watch videos." You won't see pre-video ads while you're scrolling through your friends' updates, in other words, but you might when you sit down for a viewing. The trial will start in 2018 with 6-second clips, with Facebook studying the results to see what works best for different audiences and shows.