-- Microgravity Science: Microgravity and weightlessness are best recognized by the pictures and video of astronauts floating in space, either aboard the International Space Station or performing space walks outside the station against the blue of the Earth and expansive black of space. It's often assumed that astronauts are "floating around" in this setting because there's no gravity in space. But the surprising truth is: there is gravity in space, especially directly above the Earth's atmosphere!  It's the presence of gravity that keeps astronauts, the International Space Station, and other satellites in orbit about the Earth.  The floating effect is actually a state of free fall, as described by this video:

Weightlessness (or microgravity) resulting from planetary orbit can also be achieved on Earth in short durations.  The trick fall.  Dropping an apple, some Disney Land rides, and skydiving all place objects into a state of free fall. Imagine skydiving inside of a plane, that is--the plane falls with you!  That's the concept basis for microgravity research aboard NASA's Weightless Wonder.

The parabolic flight pattern simulates the sensation of "free fall," or microgravity. Parabola is being used in this context as a shape. If you're not familiar with the term, put on your math hat and click here!

-- Bone Science
A short video describing the structure of bone and the function of bone cells: