A technique that has helped to solve many crimes in recent years is DNA evidence. DNA, or deoxyribonucleic acid, (what a word!) is the genetic material that tells cells how to develop and what to do. In other words, DNA is the stuff that determines who you are. DNA is in absolutely every cell that makes up a person's body. People leave a trail of cast-off body cells wherever they go, which means they also leave a trail of DNA. When criminals leave behind hair, blood or skin at a crime scene, they have left a DNA record of their presence.

Image courtesy U.S. Department of Energy Human Genome Program A DNA double helix

For crime scene investigators, the basic idea is pretty simple. Let's say that you have a person who is the suspect - you think he might have committed the crime you are investigating. You can get a sample of his DNA by swabbing the inside of his cheek with a cotton swab.

Now you try to gather DNA from the scene of the crime. It could come from blood, hair, or even skin cells left on a variety of things, even a cigarette butt. Then, using laboratory procedures, you can compare the two DNA samples - from the suspect and the scene -- and see if they match. If they do, there's a good chance that you have found the criminal.

Another way to use DNA evidence is to look for the victim's DNA on or around a suspect. For example, a murderer might pick up a drop of the victim's blood in his clothing.