Magnetic Resonance Imaging "sees" inside the body by mapping the position of water molecules, which exist at different densities in different types of tissue. More specifically, MRI scanners use a strong magnetic field and pulses of radio waves to manipulate hydrogen protons in the body. When the radio frequency source is switched off, the hydrogen protons reveal their position by re-emitting energy, which is captured and translated into images. MRIs are generally more expensive and take more time than CT scans, but they provide more detailed information about soft tissue. Watch the video above for a sample of some impressive MRI images of the human body in action.
Further reading: Watch the first MRI video of a baby being born