Staring at the sun for too long causes a photochemical reaction in your retinas that can kill the cells. But you can't feel it, because you don't have pain receptors back there. And once it happens, it's irreversible.
A second or two might not hurt you. It's just like staring at the sun on a normal day. But if you glance three or four times, it adds up. And permanent damage can happen pretty fast, especially for young people.
A 71-year-old guy in Portland, Oregon has been speaking out about it. He stared at a partial eclipse with his right eye for about 20 seconds in high school. And he still has a pea-sized blank spot in his vision over 50 years later. So it's really not worth the risk.