Your eyeballs actually need air to see

Your eyes are essentially scuba diving in the open air. While every other organ relies on the heart to pump oxygenated blood through a network of vessels, the cornea has zero blood vessels inside it.

Transparency is the priority here. If the cornea had veins, your vision would be obscured by a red, pulsating web. To stay clear, it evolved to absorb oxygen directly from the moisture on its surface and the atmosphere surrounding you.

This lack of blood flow makes the cornea a biological anomaly. Because there are no vessels to provide warmth or nutrients, the cornea relies on tears to deliver what it needs and wash away waste.

This is why sleeping in contact lenses is dangerous. The plastic creates a wall that chokes the cornea, preventing it from 'inhaling' the oxygen it needs to stay alive. Deprived of air, the eye starts growing its own blood vessels to compensate, which can lead to permanent blindness.

The cornea is also the fastest-healing tissue in the human body. Because it is exposed to the world, it can repair a minor scratch in under 24 hours.