Ok my teacher asked me this question a while ago, but I never got the answer.

Imagine you have two large rooms separated by a two-way-mirror (also one-way-mirror). You look through the mirror to the other room, but there's a huge mirror there and you can't see it's borders. So basically you see the mirror reflecting the two-way-mirror. If you don't take into account the  walls or floor or ceiling, what do you see? So let's just pretend you only see the mirror reflecting the two-way-mirror. I really can't explain it like it should be, but hopefully you understand what I mean.