So why did Scott Kelly "grow?"

Gravity exerts a lot of pressure on the spine. So without it, the vertebrae spread out and a person can "grow" as much as 3 inches, according to the website NASA.gov. Because arms and legs are not constructed like the spine, with space in between the bones, this elongation only happens to the spine. Scientists also think that in microgravity, the space between the bones might become filled with more fluid, as well.

NASA was able to study the effects of space travel on the spine in real time thanks to ultrasound technology that is on board the space station. Astronauts are able to scan their backs and submit the images to NASA.

Scientists first learned about spinal elongation in space from the Skylab mission, where they studied six astronauts - all of whom "grew" by 3 percent.

Here's more on what happens to your body in space (if you're on mobile, click here to watch the video):