We think that our memories are static. Once we make a memory, it stays that way for life. It accurately portrays what happened at that time.
However, experts say that this is not really true. Memories change all the time. Some experts even claim they can change slightly every time that you think of them. You are constantly "overwriting the files" that hold those memories. You're altering them.
Telling a story could change how you think about what happened. Looking at pictures could add new details that, eventually, you think you can remember. You can't. You just saw the picture, and your brain added them together. But it's nearly impossible to sort this out after the fact.
"My conclusion is that memory is what you are now," one researcher said after studying it. "Not in pictures, not in recordings. Your memory is who you are now."
That is to say, it's always changing as we change.
Why does this matter for criminal cases? It's huge. A witness's memory of an event drives their testimony. They honestly think that they remember what happened accurately. In fact, two witnesses can "remember" very different details. They will both feel convinced that they are right, and the other person is wrong.
When that witness is giving a testimony to the jury, just how much can you trust what they have to say? Even if they definitely saw the event in person, is what they remember what really happened, or has it changed over time?
This is why it's important to know your rights when you feel a testimony against you does not reflect the truth.