Originally posted by The Ori
Another theory I've heard is that because the costume is skin-tight it 'borrows' Superman's indestructability, just like Lois Lane can fly with Superman just by holding his hands.
But as for the movie's reason...in the Donner film in '78, when Clark first creates the Fortress of Solitude and then he goes on that virtual trip to Krypton with Jor-El, he returns wearing the Superman costume, so we can presume that the Fortress made it for him with Kryptonian properties.
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