Medusa is known for being so ugly that if men even look at her face, they would immediately be turned into stone statues. Perseus was somehow able to overcome this challenge of killing her by looking at her reflection in a mirror. Even though he was technically looking at her, just with a mirror, he did not turn into stone.

So how does looking at Medusa's reflection in a mirror prevent men from turning into stone? Technically he should have turned into stone as he looked at her, but this time indirectly.