For ages, people had believed the reason why they were confined to just walking on the ground was because God didn't want them hovering around in the air. Of course, people can jump up in the air, God having supplied us with free will, but inevitably we'd be pulled back to Earth, that being the natural state of things. That's why airplanes were believed to plummet down to Earth at times. In fact, airplanes' ability to fly was thought to derive from their shape being similar to a cross. The efficiency of more aerodynamic shapes would be the consequence of God having preferred a cross with a shape like that.