system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do mirrors work?

Alternatively: What makes them reflective?
gpt
Imagine you are throwing a tennis ball at a person holding a velcro pad on their hands.  If you hit the velcro pad then the ball stops and stays with the other person.  If it misses and hits their body then it will bounce back towards you.  If it misses and hits their head then it will bounce off in an odd direction (and it will be funny).

Well light and mirrors are a bit a like that.  Light is like a little ball thrown out of the sun and mirrors are like a big wall that the light hits and reflects off before hitting your eyes.

Conversely, something that is black will likely by like throwing balls at a wall made of Velcro - most of the balls will stick to the velcro and not come back to you.  

White objects, meanwhile will reflect all of the balls back off in random directions, whilst green objects will collect all the green balls, but bounce back all those of a different colour (EDIT: the other way around as pointed out below).

These balls of light are actually full of energy so they'll make the velcro like substances hot when they hit them as they release their energy into the 'velcro' object, whilst reflective surfaces won't get the energy as the ball bounces off and that is why they are cold.