Leonardo da Vinci is one of the greatest geniuses in all of human history. An inventor, artist, engineer, and biologist all in one; he was the original Renaissance man. He painted the Mona Lisa, used nature to help him come up with new scientific principles for flight and propulsion, and championed pacifism in the process. But, for somebody so smart he made some pretty dumb designs. I mean, most of these are still pretty genius for their time.

Leonardo was obsessed with flight and spent decades researching birds and scientific principles associated with lift. He designed gliders, human-propelled bird-like flying machines, and even a helicopter. But while each of these was revolutionary for their time, propulsion was an issue he never solved. Gears and pulleys were supposed to help augment a human's strength to operate such machines themselves, but there's a reason we're not birds. Some people even speculate that he wanted four people to run in a circle to power his helicopter. He especially should have known better.