MRI

The human body is largely composed of water molecules, each of which contain two hydrogen nuclei (also called protons). When subjected to the powerful magnetic field of a scanner, the magnetic moments of these protons align with the direction of the field. Next, a radio frequency electromagnetic field is briefly turned on, causing the protons to alter their alignment relative to the field. When the field is turned off, the protons return to the original magnetization alignment. This alignment changes create a signal which can be detected by the scanner, and extrapolates this information to create a three-dimensional image. This process is called Magnetic Resonance Imaging.

Cool, huh?