system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do one-way mirrors work?
gpt
A little bit of physics and a lot of marketing.

One-way mirrors are just partially reflective mirrors.  They let some percentage of light through and they reflect most of the rest.  They do this equally in both directions, which the laws of physics are actually quite adamant about.  If a mirror could be set up to only reflect in one direction (while not consuming power) then it could be used to make a cold thing warm up a hot thing, which breaks thermodynamics.

Once you have a partially reflective mirror you just make one side of the mirror very bright and the other side very dim.  This way when you're standing on the bright side of the mirror you see a very bright reflection and have a very dim view of the other side.  When you're on the dark side you see the bright room clearly while the reflection is too dark to matter.

This effect is helped along by the fact that there's a *massive* range of brightness that our eyes can deal with.  A brightly lit room can easily be hundreds of times brighter than a dim one.  That really helps the view of the dim room get washed out by the bright room.