According to the comics, Superman can fly because of a combination of powers he derives from Earth’s “yellow” Sun and from his extraterrestrial biology. On his birth planet, Krypton, gravity was much stronger than it is on Earth, so Kryptonians developed a physique that could resist its pull. When he’s on Earth, Superman’s physique can defy the planet’s weaker pull easily, and that ability, enhanced by solar energy, allows him to fly.