X-rays are a type of invisible energy wave that can pass through soft materials like skin and muscle but get blocked by denser materials like bones and metal. When doctors take an x-ray picture, they shine these special rays through your body onto a detector. The bones block the rays and show up white on the image, while the softer parts let the rays pass through and appear darker.

X-rays were discovered accidentally in 1895 by a German scientist named Wilhelm Röntgen, who noticed that certain rays could pass through objects and create images on photographic plates. The discovery revolutionized medicine because for the first time, doctors could see inside the body without surgery. Before x-rays, a broken bone might go unnoticed, or doctors had to guess what was wrong inside.

Today, dentists use x-rays to spot cavities between teeth, and airport security uses them to see inside luggage. Scientists also use powerful x-rays to study the structure of materials at the atomic level.

The word x-ray can also be a verb: when doctors x-ray your wrist after a fall, they're taking an x-ray image to check for a fracture.