A type of light — or electromagnetic wave — which is invisible to the naked eye. X-Rays are much more energetic than the light we see. They can penetrate skin very easily, for example. In the doctor's or dentist's office, X-Rays are detected on a photographic plate, allowing us to see inside the body. Like all other forms of electromagnetic radiation, X-Rays travel at roughly 300,000,000 meters per second (186,000 miles per second).
Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run twice as fast as that.
The Red Queen in Lewis Carroll's Through the Looking Glass