We often hear people say, "I have a good memory for things like that" or "I can remember names". But do you know memory is the single most important thing and it has played a very important part? For early men, there were dangerous all around them--bad weather, wild animals. The key to keep on living lay in man's ability to remember these dangers and the ways he had dealt with them in the past. In order for memories to be stored, the human brain became more complex. And as man learned how to deal with them, he became civilized  . Surprisingly, scientists today understand little about memory and how it works. They have not yet found a way to open up the brain and observe its function without destroying it. Perhaps there are two types of memory: short-term and long-term. Short-term refers to those facts that stay with us for a few seconds to a few hours. For instance, when you stay at a hotel, you remember the room number and forget as soon as you leave. Long-term memory, on the other hand, means you remember things for a long period of time. If you spend your honeymoon in a particular hotel, it's likely that you'll remember your stay quite clearly. Scientists also believe that a short-term memory can become a long-term memory through consolidation  . A lot of chemical and physical changes take place in the part of the brain. Although the human brain weighs three pounds, it contains 30 billion nerve cells   acting like a computer. When those changes strengthen the links   in certain nerve cells, a thought becomes part of the long-term memory. According to the author, without the development of man's memory there would be  _
no modern world no recorded history no animals no dangers