Did you ever have the experience of going through an old photo album or home movie you haven't seen in years, and saying to yourself, "wait a minute...that's not how it was!"
If so, you're already aware of how tricky a thing memory is. You may have an entirely vivid and seemingly reliable memory of some particular aspect of the past which those old photos prove to be false.
While these seem like oddball experiences, modern psychology is recognizing that much of our memory works this way. It's by no means the stable, objective record of events people take it to be. In fact, people's memories can rather easily be changed and new ones inserted.
That's a surprising claim to make, so let me say it again: people's memories can be changed. It's called "false memory syndrome," and has nothing to do with age or intelligence.
Here's one way false memories can be produced. Say you witness a car accident and the police ask you to describe it. Studies have shown that if the interviewing officers remain silent, you are likely to produce a reasonably accurate description of the events.
However, suppose the police ask: "After the car went through the stop sign, did the driver brake suddenly?" Whatever your answer, you may later insist you remember a stop sign, even if there never was one! A false memory has been implanted.
This kind of thing probably happens all the time. People naturally tell stories about events, inadvertently changing details as they go. Over time, we can come to believe the stories as if they were our own memories.