* Kryptonian physiology: Superman's alien biology is vastly different from humans, allowing him to withstand forces that would be lethal to us.
* Solar energy absorption: Superman's powers are fueled by yellow sunlight. This energy grants him incredible durability, making him practically impervious to bullets.
* Superhuman strength: His incredible strength allows him to manipulate his body and resist the impact of bullets.
* Aura: Some versions of Superman have an energy field or aura that protects him from harm.
Ultimately, bullets bouncing off Superman is a plot device used to highlight his power and make him a compelling character. It's not meant to be realistic, but rather a way to convey his superhuman abilities.