aao.org

5VAn9 TIL that the spots some people see while looking at the blue sky is known as the "The Blue Field Entoptic Phenomenon" and is caused by white blood cells moving through the eye.
vPjor TIL of immune privilege, where parts of your body that would be damaged by an immune response (inflammation, high temperature) work outside of your body's immune system, including your eyes and testes.
dD0a7 TIL 20/20 Vision doesn't mean you have perfect vision - it means your vision is normal, and you can see at 20ft what an average person can see at 20ft. The ideal is 20/15, which means you can see at 20ft what an average person can see at 15ft.