This puzzle is a practical one; you can solve it with pen and paper, but it's a lot more fun to try this real-world phenomenon for yourself.

Imagine you are standing before a small wall-mirror. This is no funhouse mirror; it is perfectly flat, and mounted flush with the vertical wall on which it hangs.

How much of yourself do you see in the mirror?
What if you take a few steps backward, away from the mirror? Can you see more of yourself? Less of yourself?
How big of a mirror would you need to see your entire head reflected back at you, at a given distance? How big of a mirror would you need to see your entire body reflected back at you? (Bear in mind: The mirror depicted in the art at the top of this post has not been drawn to scale.)

Credits: The original puzzle is Here at i09.

Solution: You need a mirror at least half your height to see the entire length of your body. Said another way: It is possible to see twice as much of yourself as the length of any normal wall-mounted mirror. It does not matter how far you stand from the mirror.