The police find most criminals (罪犯) because somebody tells them who the criminals are. They find other criminals by using science and their computers. When there is a bank robbery (抢劫), the police first look through their computers for the names of the bank robbers that they know about. Then they go and talk to any of these robbers who are not in prison (监狱). They ask them where they were when the robbery happened. If any of these people cannot give a good answer, the police will often use science to find out if one of them is the robber.
Many criminals leave something of themselves at the places of the crimes like a fingerprint or a hair. Or they take something away on their shoes or clothes, like dirt or animal hair. If they leave behind some of their blood or their hair or a piece of skin, it will have their DNA inside it.
Everyone’s DNA is different and their fingerprints are also different. So, if a criminal cuts himself during a robbery or leaves his hair, or even a small piece of skin, scientists can find out what his DNA is. The police can then ask the person they think is the robber to give them his hair or some blood and see if it has the same DNA. Many criminals are in prison today because of this kind of evidences (证据).
Usually the police find criminals because they use their computer.
The police do not usually ask criminals who are in prison about a crime because these criminals could not have done it.
The most useful evidences that criminals leave at the places of their crimes are things with their DNA in them.
Many criminals usually take something away only on their shoes after the crimes.
The story is about a bank robbery.

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