How do the Flip Video's work?

How does the related videos on youtube work?

• I do singing covers on youtube, and I was looking at one of my videos and in the related videos section there was a video of this kid in my school. He makes skateboarding videos which has nothing to do with singing. We didn't use any of the same music and our tags are completely different. We're not subscribed to each other or contacts. The only thing is that he goes to my school. How does youtube know that we know each other? All the other videos in my related videos section are other covers of similar songs. This makes more sense, and yet there is a random skateboarding video of this kid I know. Also I have watched his videos before, so I checked on a different computer that I have never watched his videos from, and I wasn't signed on any account, and yet it was still there. How does this happen?

• Answer:

  Maybe because you've watched his videos before..? I'm not sure to be honest with you :/