You're probably more familiar with his other work -- namely the Mona Lisa. But before Leonardo Da Vinci painted her enigmatic smile, he honed his skills sketching the most minute details of the world around him --- with a particular emphasis on birds and flight.

Now, a rare opportunity to peek into the his journal which is written in a special backward code. It's on loan to the National Air and Space Museum in Washington from Italy.

Curator Peter Jakab.

SOUNDBITE: Peter Jakab, Chief Curator Of The Smithsonian's National Air And Space Museum Saying:

"Da Vinci items don't even travel around Europe very much, to have an original Da Vinci item here at the Smithsonian is an extraordinary opportunity, and particularly special to have this particular codex, which is about flight of course, in the same space as the original Wright Flyer, one of the signature objects of the Smithsonian of course is the original Wright brothers airplanes, so to have Leonardo's ideas about flight and then the airplane which they were manifested in the same space, was a once in a lifetime opportunity."

The 18-page collection --documents the principles of movement and mechanical flight as observed through birds. It was also the inspiration for the ornithopter -- a mechanism he hoped would allow humans to soar. The exhibit is in Washington for the next 40 days.