How does this work? The outer layer of skin contains many microscopic pores that secrete sweat and oils. Sweat is mostly water, but it contains a very small fraction (1.5%) of salt, amino acids, and proteins. These chemicals remain on the skin after the water evaporates. The skin also contains sebaceous glands, which produce oils. Although the fingertips contain few sebaceous glands, the face and head contain many and people touch their faces and hair often, transferring oil to the fingertips. The oil and the residual chemicals from sweat cling to the surface of the fingers and attract dirt and other substances such as cosmetics and grease from foods and oils. Whenever a person touches something, these residues are transferred to that surface. Since people rarely commit crimes without using their hands, the prints from their fingers are often left on surfaces at the crime scene.