magine you are asked to remember what you were doing exactly one week ago. You would probably have to make quite a mental effort to sift through your memories. On another occasion, a smell, a picture, or a word might suddenly and unexpectedly trigger a vivid memory of something that happened to you. Science still does not fully understand why our brain sometimes automatically supplies us with a memory that we have done nothing to deliberately call to mind, whereas why, on other occasions, we cannot remember things even though we make efforts to recall them.