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Interesting question, actually. "Locard's Exchange Principle" says that when two things come into contact an exchange is going to happen. What this means for evidence and crime scenes is that when a criminal goes into a place to commit a crime, he/she will both leave something as well as take something way in terms of evidence as a result of having physical contact with the scene.
Think of it this way...when he goes to the scene he is going to potentially leave footprints, hair, skin cells, fingerprints, clothing fibers, and all sorts of other things. When he leaves the scene he is going to take with him things specific to that area: pollen, dirt stuck to shoes, plant material, and that kind of thing. It is the detective's job to seek out this type of evidence by understanding that it is always there, someplace.
Locard, the great-grandaddy of crime investigation, knew that highly trained individuals would have a chance to help solve crimes by collecting this evidence.
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