To put it simply, Synesthesia is a condition that some people have where the different senses—touch, taste, hearing, vision, and smell—get mixed up instead of remaining separate. The most common variety is called Lexical Synesthesia, which is where letters and numbers each have individual colors. For instance, someone with this condition might say that the letter "A" has a sunflower yellow tint with a crumbly feel to it. The number 2 might be the color of wet cement.

There are other pairings, like sound-to-vision, where the notes of the violin could cause the listener to see small silver balls raining down in front of them. For others, their synesthesia takes the form of sound-to-taste. The word "cat" might taste like peanut butter, or the name Michael might be hot buttered popcorn. These perceptions feel very real to the person having them.

Most people who have Synesthesia think that surely everyone else sees the same things they do. At some point they inevitably find out that's not the case—usually by the blank stares they receive when they talk about it. Books like A Mango-Shaped Space are getting the word out about this fascinating condition. Scientific researchers have known about it for over a hundred years, but only recently has it been getting the attention it deserves. One out of every two thousand people are now believed to have some form of Synesthesia, and the numbers may be even higher than that. So go on, ask your friends and family what color "A" is, and see what they tell you!