Seriously. How do they fly?
It's not because Krypton had more gravity. That doesn't make a lick of sense.
I've heard the telekinetic theory, and it seems to fit best. You know, how Kryptonians develop minimal telekinetic powers that enables them to fly and lift objects without breaking them (for example,Supes could lift a plane from the wing, even though in real life, the wing would snap off.)
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Well, its widely known that they have enhanced musculature. This allows them to tighten their abdomen to such great tension that their flatulence leaves their bodies at such pressure that they are propelled along at whatever speed they want.