ELI5 One Way Mirrors - how do they work?

How do one way mirrors allow light to pass in only one direction? If light travels through the glass equally from both sides, why does the simple flip of a light switch create a hidden observer? How does a microscopic layer of aluminum or silver on a piece of glass actually bounce light back to create a reflection? Why does our brain instantly prefer our flipped mirror image over the real perspective captured in a photograph?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: avalancheofneed, afropuf, coolminded, tequilaamocking_bird, blade_torlock, imtrollin_theyhatin, antiproton and spilgrim16

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