There's even documentation as to his invention. Also, you would think that his incredible powers would be a clue. Let's take the ability to fly as an example. In earlier writings, he is described merely as able to leap up and into the air to air himself to where he wants to go. But later, we find that he is able to hover at will, and even change direction in the vacuum of space. His power of flight has evolved into something less likely according to known physics. All of this peculiarity should be reminder enough that Superman is a character in stories written by ordinary people, and not an actual being.