This Monday, people around the world will turn their telescopes towards the sky to catch a glimpse of Mercury as it transits across the sun. (mental_floss will publish a full article on Sunday on how, when, and where to watch the rare celestial event.) But anyone hoping to see Mercury—and anything afterwards—should remember to take some safety precautions, because without protective solar filters, staring directly at the sun through a telescope can have some pretty drastic consequences.

We were all told not to look directly into the sun as kids, but that warning is especially important when it comes to telescopes. In the short video above, astronomer Mark Thompson reveals exactly what kind of damage the sun can do to our eyes, holding a couple of pig’s eyes purchased at his local butcher’s shop to the eyepiece of a telescope as an experiment. The video, which is certainly not for the squeamish, shows the eyes beginning to smoke and burn within 20 seconds. Check out the fascinating, and horrifying, experiment above.

[h/t Popular Science]

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