X-Rays pass electromagnetic waves of light through the body and onto an electrically charged metal plate. Since the magnetic waves will pass through some things easier than others (Going through tissue/flesh is easier than bone and going through bone is much easier than going through metal.) the level of waves that make it through to the charged plate varies, thus leaving various levels of charge on the plate. During the development of the x-ray, a black powder is applied to the plate. The higher the level of charge in a particular area, the more powder adheres to that section. Because of this process, you will be able to easily see the differences between tissue, bone and the little metal monopoly piece that Junior swallowed.