Its those things that just pop up on your brain at any point in time. It can be just going to a place or seeing a movie. I believe the brain has something like folders in it. These folders go from A-Z and has every one of your memories in it good or bad.. Like your first day of school or a high school dance. Even the memory of your first time whether it was a good time or a bad time. Or your first heartbreak but without memories life would be dull. So sometimes if you think you would like to not remember some things that happened just think about all the good things that happened also.