How common sense can lead to blindness
I'm a big nerd who likes astronomy and science in general. When I wanted to look at the sun, my common sense told me "Sunglasses block harmful UV light and visible light. So I'll just take three pairs of sunglasses, put one in front of the other, and through this triple pair I can safely look at the sun for long periods of time. I'll know my eyes are safe as long as they don't hurt."
Fortunately, I don't trust my common sense much when there is risk of death, blindness, paralysis, losing limbs, or some other serious permanent injury. In searching the internet for a safe way to look at the sun, I found this and this article which explains that while sunglasses block visible and UV light, they don't block enough infrared light. So even if you look at the sun through multiple pairs of sunglasses, the lens in your eye will focus invisible infrared light onto the most sensitive part of your retina, causing permanent damage. The insidious thing about it is that you won't feel any pain or discomfort when this happens because the retina cannot feel pain.
Ideally you'd watch the sun through a camera obscura or some other device that allows indirect observation. But if you must look directly at the sun, the only safe way is through filters that are designed especially for this purpose. I wouldn't trust any home-made or improvised filters like exposed camera film, CDs, or magnetic computer disks.
Boy goes blind on his way home from grocery store
BROOKLYN, NY - A boy was regularly given money by his father for light grocery shopping. Occasionally he would spend a few quarters on video games during his shopping errands. He was not permitted to do this but he got away with it often enough. One day he got carried away and spent most of the grocery money playing a popular video game with his friends. He knew he was in big trouble because not only did he play video games while shopping which was bad enough in itself, but he spent his family's food money on prohibited entertainment. If he returned empty-handed and told the truth, his father would throw a fit and severely punish him. So he and his friend decided to come up with a story... he would say that he was robbed by some other boys. On his way home, the boy decided to get into character and act as if he were just robbed. For realism, he wanted to be crying, but how could he make himself cry? His friend suggested that he stare at the sun until his eyes began to tear up. The plan worked. The boy never saw any punishment... nor anything else for the rest of his life.