Grown-ups are often surprised by how well they remember something they learned as children but have never practiced ever since. A man who has not had a chance to go swimming for years can still swim as well as ever when he gets back in the water. He can get on a bicycle after many years and still ride away. He can hit a ball as well as his son. A mother who has not thought about the words for years can teach her daughter the song that begins with"Twinkle, twinkle,little star" or remember the story of Cinderella or Goldilocks. One explanation is the law of over learning, which can be stated as follows: once we have learned something,additional learning trials increase the length of time we will remember it. In childhood we usually continue to practice such skills as swimming, bicycle riding, and playing baseball long after we have learned them. We continue to listen to and remind ourselves of words such as "Twinkle,twinkle, little star" and childhood tales. We not only learn but overlearn. The multiplication table(乘法表)is an exception to the general rule that we forget rather quickly the things that we learn in school, because it is another of the things we overlearn in childhood. The law of overlearning explains why cramming for an examination, though it may result in a passing grade, is not a satisfactory way to learn a college course. By cramming, a student may learn the subject well enough to pass the examination, but he is likely soon to forget almost everything he learned. A little overlearning, on the otherhand, is really necessary for one's future development.