A human fingerprint is a unique pattern of ridges and valleys on the skin of the fingers and thumbs. Fingerprints are formed during fetal development and remain unchanged throughout a person's life. They are used for identification purposes because of their uniqueness and permanence. Fingerprints are made up of a series of ridges, which are separated by valleys. These ridges can vary in thickness and direction, and can form various patterns, such as loops, whorls, and arches. Fingerprint patterns can also be used to determine a person's sex and race.