The problem with memory is that it's not etched onto an unchangeable physical medium. Your brain operates on vague notions and occasional sharp details that stick in your mind, somgetimes for no apparent reason (like how you can never forget your third-grade teacher's weird earlobes, but can't remember what grade you got). The problem with that system is that it is incredibly easy to manipulate. If someone offers your brain a reminder of those fuzzy details -- say, a photograph, or just a very convincing person telling the story -- it will latch onto it and make you remember things that way.
All that matters is that the version you're given is similar to something that could have happened, or even better, seems like it should have happened. Like that famous photo with the tanks, and the guy, and the crowd of protesters. See, there actually wasn't a crowd. It was photoshopped in.
Experiments found that when showing people the crowd photo, they were much more likely to remember seeing crowds in their memories of watching it on television. Keep in mind, for people of a certain age, we're talking about an image they may have seen 5,000 times at various points in their life. One plausible piece of doctored evidence, and all those memories were rewritten. So your brain is less like a hard drive and more like a sandy beach people write shit on with their fingers.