What is the passage mainly about? [br] [originaltext] (12)Memory, they say,

(12)Memory, they say, is a matter of practice and exercise. If you have the wish and really make a conscious effort, then you can quite easily improve your ability to remember things. But even if you are successful, there are times when your memory seems to play tricks on you. Sometimes you remember things that really did not happen. One morning last week, for example, I got up and found that I had left the front door unlocked all night, yet I clearly remembered locking it carefully the night before. Memory "tricks" work the other way as well. Once in a while you remember not doing something, and then find out that you did. One day last month, for example, I was sitting in a barber’s shop waiting for my turn to get a haircut, and suddenly I realized that I had got a haircut two days before at the barber’s shop across the street from my office.(11 - 1)We always seem to find something funny and amusing in incidents caused by people’s forgetfulness or absent-mindedness.(13)Stories about absent-minded professors have been told for years, and we never get tired of hearing new ones. Unfortunately, however, absent-mindedness is not always funny.(11 - 2)There are times when tricks of our memory can cause us great trouble.