Today is Superman's 70th birthday. As pretty much everyone on Earth knows, the character Superman is an alien from the lost planet Krypton, who gains super powers on Earth because of its yellow sun. Needless to say, this makes absolutely no sense. If the color of sunlight gives you superpowers, so would a string of colored Christmas-tree lights.
Which is not to say that an alien on Earth couldn't be superhuman in various ways. A creature from a high-gravity world would indeed be tremendously strong compared to ordinary humans. It would also be able to jump around like an astronaut on the Moon. This is about the level of super-ness that Superman exhibited in his original incarnation, created by science fiction fans Jerry Seigel and Joe Shuster.
But later comic writers kept upping Superman's power level, giving him the strength to lift battleships or skyscrapers, the ability to fly, nearly complete invulnerability, and, of course, "X-ray vision." To compensate, they also gave Superman a weakness to Kryptonite, a mineral from his home planet. Kryptonite meteors began landing on Earth with remarkable frequency.
Surprisingly, all this can actually be made to make sense scientifically. The key is the Kryptonite. For so many meteors from Krypton to land on Earth, the lost planet's explosion must have filled this part of the Galaxy with debris. That means Krypton itself must have been extremely massive. Really extremely massive. How massive? Well, we assume it didn't collapse into a black hole, so the "planet" Krypton would have been a neutron star or quark star about four times the mass of the Sun.
Suddenly, it all makes sense. A humanoid like Superman capable of standing and walking on a neutron star would have to be made of degenerate matter himself. Now we understand why he's bulletproof -- he's effectively a single giant atomic nucleus shaped like an ace reporter. This also neatly explains his strength and even his flying ability. To a being used to conditions on the surface of a neutron star, it would be difficult to even notice terrestrial gravity. Neutronium eyes also explain how he can see x-rays.
This all works out quite well, except for one little problem: Superman's weight. A man-sized blob of neutron-star matter would have a mass of about 100,000 tons. To such a being, the solid surface of the Earth would be like water -- just standing still he'd sink into anything, even hard rock. He couldn't even swim in rock, any more than a human can swim in air. He'd sink down to the core of the Earth and remain there, fighting a never-ending battle for truth and justice in the liquid iron.
So, to Clark Kent, alias Kal-El, in his comfortable refuge 4,000 miles below our feet, happy 70th birthday!