Normally, peoples' memories of things they have learned are most clear immediately after they have learned them. People will then forget more and more knowledge as time goes on. After a few months they may only be able to recall a small percentage of what was initially learned. This makes recalling information difficult when it needs to be done. If you review knowledge frequently, however, then you will be able to keep it fresh and alive in your mind. This makes it easy to recall, with minimum effort, when you need it.