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- Xylitol: a naturally occurring sweetener, that does not cause decay.
- X-ray: X-rays were discovered by physicist Wilhem Conrad Roentgen, November 8, 1895. X-ray is derived from the German, X-strahl ("X" designating unknown, and strahl meaning "ray or beam"). Today x-rays are recognized as important diagnostic tools to visualize areas inside bodies / objects that are otherwise undetectable. In dentistry x-rays are used to detect cavities, and also for examining the supporting jawbone and surrounding structures.