Leonardo da Vinci is often called one of history's greatest thinkers. Imagine the world of 500 years ago. At that time, few people were inventing new things. Bulent Atalay, who wrote a book about Leonardo, says, "He invented the future." Leonardo studied engineering and physics. He was also interested in the science of flight. With all of that knowledge, he imagined flying machines -- 400 years before the first airplane!