There are some mirrors which allow to see through on one side while seeming like a mirror on the other side (Seen it in any James Bond film?). These may be very useful when you build your house. But when these kinds of glasses are kept in improper places, you may be being cheated. There might be someone behind you watching you. So how do you know whether you are watching into a genuine mirror or a mirror-cum-glass one?
I would be cheating if I put the solution here, because I got this from another blog. So I will post the link : Bioinformatiz