The title is borrowed from this exciting Scientific American article. I don't know if the idea of Synesthesia is as exciting for everyone as it is for me. I understand it can be very distressing for the synesthetic person but its milder forms can be really interesting without being patently disadvantageous.
I came across this article a few days ago when I was reading about LSD (no, don't get any ideas.. it was perfectly innocuous websurfing driven by nothing more than normal scientific curiosity). I knew a little about synesthesia earlier but always thought its a permanent genetic disorder (which it is, in some cases). I didn't know that it can be induced by drugs like LSD. Anyway, I'm going off topic. Whats most interesting about the sciam article is not the induced synesthesia (where one might hear colors and taste shapes) but the clever experiments done with permanent synesthetes, specifically with people who have a condition that makes them see a particular numeral always in a related color. For example, if they see a "3" (even if typed in normal black font), they would see it in red hue. similarly, other numerals might be related to other hues. Clearly, this is not just a "cross-connection" between signals of different senses as one might expect in someone who sees red when hearing a metallic sound (with intesity of the color related to the loudness of the sound). What is special about "3"? This seems to indicate that the cross-talk happens not between signals but between the processed information. But wait!! What is even more astounding is that the "3" appears red even if the person can't see it clear enough for him to process it as a representation of number three. Confused? Read the article.
The article ends with a nice hypothesis that "creativity" is actually a manifestation of extremely mild (and universally present, though in different degrees) synesthesia. Confused again? As I said, read the article. :)