I went to the gym the other day and was changing in the women's locker room when I noticed two little girls observing me in a mirror and giggling. Lockers were obscuring the direct view between us, and the girls seemed to assume that they are the only ones spying. They were shocked when I waved back at them. They hid, then came back to check whether I am still in the mirror, whispering to each other, and excitedly retreated again.
The mathematics of mirrors is one of the most elegant daily math puzzles we encounter. Flat mirrors do a perfect reflection: every single ray of light is bouncing back at exactly the same angle it came in. If I can glance at you in a mirror and see you, you can look back and see me. Our glance goes back-and-force along the same ray, symmetrically reflected by the mirror, making it a non-ideal spying tool. This all is also applicable to your car's rear view mirror. You can see your kids and they can see you. So, no nose picking!
Mirrors a full of magic. Here is a previous story from The Math Mom that sheds light on other mirror mysteries: Mirror Magic Revealed- why mirrors flip Left-Right and not Up-Down, and why ambulance sign is confusingly flipped.