Okay, this one is for the stalker in all of us. How many times have you knocked on a friend's door, and wish you could see what's happening through the peephole? Well, look no further. Just don't blame me when you get busted for being a looky-loo.
Just place the viewer over the peephole and you can see who is looking at you. Now you are prepared to see what you are about to enter.