You've likely seen MRI images like the one below. The technology that first made them possible has completely changed the way we do neuroscience. Where do those images come from? How does Magnetic Resonance Imaging work?

Dwayne Reedt en.wikipedia derivative work: Sherool [CC BY-SA 3.0] or GFDL, from Wikimedia Commons

In order to explain MRI, we only need to explain three things - why a compass points north, how an opera singer can shatter a glass, and the magic of radio.

1. Magnetic
Protons, one of the basic subatomic particles that make up everything, have a slightly positive charge. You know how the Earth's magnetic field is strong enough to orient a compass needle north? This also happens with the protons in your body, except instead of just pointing, they also spin - around 2000 times a second. What happens if we put your body in a powerful magnetic field, a field many times more powerful than the Earth's? The protons align and spin a LOT faster. 

2. Resonance
When you tap a wine glass with a spoon, the sound you hear is the natural frequency of the glass. If you can duplicate that sound with your voice, you will cause the glass to resonate. If you do it loudly enough, the glass will shatter.  MRIs use this exact same principle, but lucky for us, the protons in our body can't shatter. By producing a frequency that matches the new, extremely fast spin of the protons induced from the magnetic field, we can force the protons to realign, creating a resonance, just like the opera singer with the glass.

3. Imaging
Finally, we turn off our resonator. This causes our protons to realign with the magnetic field and spin, giving off a weak radio signal. If we record this radio signal from three angles, we can pinpoint, in a three dimensional "voxel" (1mm each side, usually), where that signal is coming from. Different substances decay in signal strength faster than others, so we can use that differential to get contrast in our images. Feed this data through a powerful computer, and voilà, a three dimensional, structural MRI scan!

Thanks to Dr. Stefan Hetzer for the wonderful metaphors.