Its something that has giving you an emotion response and often when you feel that emotion the image of what happened at the time resurfaces.
Mmmm, I love cake. Especially chocolate cake. I could eat chocolate cake all day. It's yummy with ice cream..Hey, ablford, you're driving me crazy with all your talk about cake, Now cut that out! A nice big piece from the middle. Got any ice cream? Damn, I just cant stop thinking about that chocolate cake your gonna bake me....
if a person or a person stirs a feeling in us; it will make it hard to forget. It stays with us for long time. But as time passes; the image starts getting blurry and it will be hard to see it clearly.
Sometimes a mental image can make it hard!