Dr. Emond Locard (1877-1966) was a pioneer in the field of forensic sciences. He speculated that you exchanged physical materials every time you made contact with another person, place, or thing. 

He believed that no matter where a criminal goes or what he does, by coming into contact with things, a criminal can leave all sorts of evidence, including DNA, fingerprints, footprints, hair, skin cells, blood, bodily fluids, pieces of clothing, fibers, and more. At the same time, they will also take something away from the scene with them. 

Just think: when you enter a room, you leave behind trace DNS evidence - skin cells, a piece of hair, or a tiny bit of saliva. Also, your shoes will contain trace evidence - the dirt on them, the shoe print, the wear pattern, and shoe size. 

You can read more about Locard's Exchange Principle in this howstuffworks.com article.