In 1885, Hermann Ebbinghaus studied his own memory. He wanted to know how quickly he forgot new information. He gave himself tests after different periods of time to see how much he forgot each time. The result is called The Forgetting Curve.
 We forget the fastest in the first 24 hours.
 And then we forget more and more but the speed slows down over time.
 After about six days, we have forgotten most of the new information.
 If we revise the information …
 ... we make the memory stronger and we slow down the speed of forgetting.
 If we revise again and again, we can leave longer and longer periods of time between revisions.
Time is not the only thing that makes us forget something. If we are tired or under stress, we forget faster. If the information was difficult to understand we will forget it faster too. But things that are very important or meaningful to us are easier to remember.