The police find most criminals because somebody tells them who the criminals are. They find other criminals using computers and science. When there is a bank robbery, the police first look through the computers for the names of the bank robbers they know about. Then they go and talk with these robbers who are not in prison. They ask them where they were at the time of the robbery. If any of them cannot give a good answer, the police will use science to find out if he or she is the robber.
Many criminals leave something of themselves, like a fingerprint or a hair at the place of the crime. Or they take something away on their shoes or clothes, like dirt or animal hair. If they leave behind some of their blood or a hair or a piece of skin, the police will get their DNA.
Everyone’s DNA is different, and in the same way their fingerprints are different. So, if a criminal cuts himself during a robbery or leaves a hair or even a small piece of skin, scientists can find out what his DNA is. The police can then ask the person who they think is the robber to give one of his hairs or some blood and see if it has the same DNA. Many criminals are in prison today because of this kind of evidence.