X-rays are a type of high-energy radiation. An X-ray machine produces bursts of X-rays which can easily pass through fluids and soft tissues of the body. However, bones and other dense tissues will block some of the X-rays. The more dense that a tissue is, the fewer X-rays that will pass through it. Because the particles which make them are not held closely together, air and water are less dense and more X-rays are able to pass through them. In an X-ray picture, air shows as black or darker areas, whereas bones show as light/white areas. The other lighter area in the middle of the image is the heart, as the muscles which make up the heart are quite dense and stop many of the X-rays from passing through.