Fingerprints have been used for over a hundred years to positively identify criminals. You can leave a fingerprint on almost everything you touch. Because everyone's fingerprints are unique, they are of great use to detectives.

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To find these prints, crime scene investigators dust an area with special powder. This powder sticks to the oil put down by the finger ridges, revealing the fingerprint. Sometimes, investigators also spray a chemical that reacts to the oil-coated areas.

Then, the investigators photograph the visible prints or lift them with a piece of tape, and scan them into a computer. The computer compares them to millions of fingerprints already on record. (The FBI has almost 100 million people's prints on file!).

Not everyone is in these databases because not everyone has been fingerprinted, but millions of people have been. For example, anyone who has ever been sent to jail or prison has his or her fingerprints in the system. It is easy for the computer to match a fingerprint from the scene of the crime with a fingerprint in the database, if it exists. They can also match the fingerprint directly with the prints of the suspect.