You decide to send a special message to your sweetheart. On a piece of poster board you carefully letter “I LOVE YOU SO MUCH!”

With a camera in one hand and your poster in the other, you look in a mirror, pucker your lips in a kissing pose, and snap a picture. Only then do you notice that in the mirror your message reads “!HCUM OS UOY EVOL I”

Not only is the sentence backwards, the words themselves are backwards. And what is more, some of the letters (C S E and L) are backwards.

“Darn it,” you say. “Why do mirrors do that?”

A better question is, how do mirrors do that? The answer may surprise you. Mirrors don’t turn things backwards, we do.

On a piece of paper, draw an arrow that points to the right. Turn the paper around horizontally, the way you would to look at it in a mirror. The arrow, which is now on the side of the paper away from you, is pointing where? To your left.

Why is it pointing left? Because you flipped the paper horizontally. As you turned the paper, the tip of the arrow moved from the right, to forward, to pointing left.

Set the paper down in its original position and write the word TOOL above the arrow. Flip the paper horizontally. Not only is the arrow now pointing to the left, the word TOOL is backwards. The word now spells LOOT, except the L is backwards. In flipping the paper, you flipped the word.

“But,” you say, “a person facing me can read the paper when I flip it horizontally, why can’t the mirror?”

We read from left to right. When you flip a message horizontally so that the other person can see it, it goes from being left to right on your side to being left to right on their side. So they can read it.

A mirror doesn’t show what a person facing you sees, it shows what the paper looks like from your perspective, and from your perspective, the words are backwards. It shows what you would see if you had X-ray vision and could look through the paper.

Test this by writing on thin paper with a black marker. Turn the paper around and look at it in a mirror. Compare what you see there to what you see when you look through the paper.

Some ambulances have ECNALUBMA (with the B, L, N, and C backwards, which I can’t show here) lettered on the front. That way, when the driver of the car ahead looks in her rear view mirror, she reads AMBULANCE.

So what do you do with the message to your sweetheart? Either write it backwards and take another picture, or send a note with the original photo that says, “For a special treat, look at this picture in a mirror.”

Mirrors don’t flip things horizontally (along the x axis) and they don’t flip them vertically (along the y axis). They do, however, perform a weird sort of magic and flip things along the z axis.

I’ll explain next week.