I am responding to popa, taking up his offer to respond to the 100k/90k riddle on the Age Gap thread thereafter:
Why when you look in a mirror, is the right and left inverted but not the up and down?
When the plane of the mirror is horizontal, the mirror does reverse up and down. Just think of the upside down landscape you can see in the surface of a lake.
Left and right are the only directions which are described relative to the observer. Up and down (and north, south, etc) are independent of whichever way you’re facing. When you look in the mirror, up, down, east and west are still in the same place. But you read printed text from left to right – not east to west – hence the problem with ‘mirror writing’.
A mirror doesn’t reverse left to right either as you will see with an asymmetric object. Imagine the mirror is one of those impression taking toys that had a dense number of pins in which are pushed back on contact and retain the 3D shape. Now imagine pressing your body into it and step back. The illusion is, because we are roughly symmetrical, that there has been a left/right inversion. The reality is your right side is on the right side of the image.
One way to start to get a handle on this is to take a piece of paper with your name written on it. Look at it in the mirror and it will be reversed. Now turn it vertically. You will find that the lettering is reversed, but it is not upside down. That should start to give you a clue as to what is going on. Now take your finger and point to the first letter in your name, both on the paper and on the mirror. Notice that there is a one-to-one correspondence — if the letter appears on the left hand side of the paper from your perspective, it appears on the left hand side of the mirror.
Take a piece of thin, translucent paper and write your name on it. Stand in front of a mirror and hold the paper up so that you can read the paper normally. Now look in the mirror. You are seeing the back of the translucent sheet in the mirror, and the word is not reversed — it looks completely normal. Now turn the paper over and look at it in the mirror. It is reversed, but so are the letters on the back of the translucent sheet. Note that you turned the paper over — you reversed it!
In other words, the reason why letters look reversed in a mirror is because you are presenting them to the mirror reversed. The reason why someone coming toward you can read the message on your T-shirt is because the letters on your T-shirt are reversed (from your perspective) when you wear it. If you printed the words on the T-shirt so that you could read them (from the inside of the shirt), the words would look reversed to an approaching person but they would look great in a mirror.