This method is called bullet matching. When a bullet is fired from a gun, there are tiny microscopic scratches left on the bullet. Every gun is completely unique, and no two guns leave the exact same pattern on the bullet. Forensic scientists then fire a bullet, and compare it to the one found at the crime scene.
If the two bullets match, then they know that gun was used to commit the crime. Another type of evidence is imprint evidence. Some examples of imprint evidence are shoe prints and tire prints. Tire prints can be especially useful. From a tire print, forensic scientists can gather enough information to tell which way they went, what kind of tires they have, and most importantly kind of car the person was driving. Of course, in order to gather all this information, investigators need to take photographs of the crime scene.
First, they take a picture showing where the crime scene is, and take pictures of the areas around the crime scene. Next, they photograph the whole room in which the crime was committed. They take pictures of any possible evidence; like bullet holes, items dropped, or footprints. They also take pictures showing how big the evidence is, by putting a ruler next to the evidence and then photographing it. Video recorders are also used occasionally, to take long shots of the whole crime scene.