The one rule of the solar eclipse was to not look directly at it.
This is because you could permanently damage your eyes by burning your retinas.
There are living examples of people who have done this, like Louis Tomososk, now-70, who burned his retinas looking at the solar eclipse in 1963, age 16. He's had a hole in his retina and a blind spot ever since.
David J. Calkins, vice chair and director for research at the Vanderbilt Eye Institute, told the New York Times:
We don’t have pain receptors in the retina, so you won’t even know the damage is occurring.
It can range from blurry vision to absolute permanent vision loss.
United States President Donald Trump, you'd hope, would lead by example in not sneaking a peek.
Mr #DonaldTrump watching the eclipse. https://t.co/ev7a6qbwdI