That question was asked by . . . somebody. On some TV show I watched. I don’t remember the specifics. But the basic thrust of the question was, how exactly does he fly? He has no propulsion, no jet pack. He doesn’t leap across distances, like the Hulk. One second he’s standing next to you, the next he’s up, up, and away. How does he fly faster, for that matter? I never got into Smallville; maybe the writers explained it on that show.
But it doesn’t matter how Superman flies. He just does. Because the writer says so. Because it’s fiction, and in fiction, you can do any damn thing you please. You can suspend disbelief, dangle it from your fingertips like a puppeteer. But you have to be fair. You can’t have a character who can turn invisible in one book, and then, in the next, he can also walk through walls,with no prior explanation.
If you want a good explanation of fairness in fiction, check out Stephen King’s Misery. Annie Wilkes goes off on Paul Sheldon, after he resurrects his fictional character in a way that, to side with the psychotic nurse, really is unfair.
I try to be fair in my work. I’ve created a rigid set of rules for my werewolf character, and I stick to it. Deviation is bad. Deviation leads to the un-suspension of disbelief, the cutting of the puppet strings. No writer wants that.
Also, I don’t want someone like Annie Wilkes coming after me. Even if she is my number one fan.