[awongh]

I recently learned this and it blew my mind because it had never occurred to me.

Another way to put it is that when you’re in a moving bus and you throw a ball towards the front of the bus, the ball is moving the speed you threw it plus the speed of the bus, but, when you shine a light, the photons from the light are moving the same speed as someone off the bus! That seems super weird.

[davrosthedalek]

Exactly. Now build a clock by bouncing the light between two mirrors, and think about how the clock looks like in the bus, and outside of the bus. -> special relativity.