Superman can shoot beams from his eyes, fly faster than a jet and survive point-blank gunshots to the head because our sun is a different color from his sun - fine, we'll accept that. He's essentially a solar battery, constantly absorbing energy that turns him invincible, superhumanly fast and all that other stuff that makes him one of the hardest characters to write.
But, even though his skin is impenetrable, what about his innards?