Here is how TikTok videos get featured on the For You page, according to the platform. When a video is uploaded to TikTok, the For You algorithm shows it first to a small subset of users. These people may or may not follow the creator already, but TikTok has determined they may be more likely to engage with the video, based on their past behavior. If they respond favorably—say, by sharing the video or watching it in full—TikTok then shows it to more people who it thinks share similar interests. That same process then repeats itself, and if this positive feedback loop happens enough times, the video can go viral. But if the initial group of guinea pigs don’t signal they enjoyed the content, it’s shown to fewer users, limiting its potential reach.

Recommendation engines like the one TikTok uses have also helped power its Silicon Valley competitors—and over the years, their algorithms have been faulted for amplifying hateful ideologies and creating echo chambers. YouTube, for example, was criticized for adjusting its recommendation algorithms to increase time spent, because the decision may have helped content like conspiracy theories thrive. TikTok says the For You page algorithm isn’t optimized for any specific metric, but rather is designed to take into account many factors, like whether people like using the app and if they choose to post content themselves. User safety is also considered: TikTok says it may block videos depicting things like “graphic medical procedures or legal consumption of regulated goods” from being recommended, because some people may find the content shocking.