X-rays are a type of high-energy radiation. An X-ray machine can produce short bursts of X-rays. The rays pass easily through fluids and soft tissues of the body. However, dense tissue such as bone will block some of the X-rays. (Density means how much of something there is in a certain amount of space.) The more dense the tissue, the fewer X-rays pass through. Air and water are less dense because the particles which make them are not held closely together. They let more X-rays pass through them.
This is a companion discussion topic for the original article at https://patient.info/treatment-medication/x-ray-test