Here's a (cheeky) puzzle for you...

[Tiberius]

A mirror flips images around horizontally, but not vertically. Why? For instance, if you wear a t-shirt with the words "Atheist Forums" on it in front of a mirror, in the mirror image, the letters will be reversed but will not appear upside-down (nor will anything else).

[jonb]

What is on your left is also on your left in the mirror, etc.

[pocaracas]

You're probably looking for one of these:

[Darwinian]

Best guess: Because the light reflected is the opposite of the angle at which it enters.

[Napoléon]

Magic.

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If I had to guess it would be something to do with how your brain flips the image your eyes receive?

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[Something completely different]

lol internet.

http://en.wikipedia.org/wiki/Mirror_image

[Tiberius]

Answer for those interested:

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I did say it was a "cheeky" question, and indeed it was. jonb got the general gist of it, but I thought I'd add an explanation.

Nothing is actually "flipped" in a mirror image. If you place two objects in front of a mirror and stand behind them, the object on the left is still on the left in the mirror image, and the object on the right is still on the right in the mirror image. Likewise, the top of each object is still at the top in the mirror image. No flipping occurs either horizontally or vertically.

Well, wait a minute, what about the words "Atheist Forums" on a t-shirt? Stand in front of a mirror with anything written on your t-shirt and it will appeared flipped horizontally in the mirror. What gives? Well, here's the trick question bit...the mirror didn't do any flipping...you did! By facing the mirror, the words are already flipped (the 's' of "Forums" is on the left, and the 'A' of "Atheist" is on the right), and they get reflected back exactly in those positions. Indeed, even the letters themselves have been flipped prior to being reflected. When facing the mirror, the top line of the 's' (which ends on the right) is on the left of the t-shirt, and the bottom line of the 's' (which ends on the left) is on the right of the t-shirt. The mirror simply reflects back these positions, leading to the appearance that the letters are flipped and that the mirror has done the flip.

To illustrate this best, I wrote the words "ATHEIST FORUMS" on a transparent piece of plastic:

Held up to a mirror so that the words are the correct way around (from behind the mirror), we can see that the reflection in the mirror shows them the correct way around also:

Flip the words (as they would be on a t-shirt) and we can see that the reflection mirrors them exactly as they should be:

Science!

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By the way, I got this question from the alt-text of this XKCD comic: http://xkcd.com/1145/

[5thHorseman]

I approve this message.

[downbeatplumb]

Fire engines have the words "fire engine" written in reverse on them so that when you see it in the rear view mirror its the right way round.