Synaesthesia is something very difficult to explain to people who don't have it in one way or another themselves, and I only became aware of what it was a few years ago. Until I knew what it was, I always assumed that other people felt songs and music had "colour" and that numbers and even time periods were percieved in ribbons and shapes.
While not everyone percieves the world in this way, many other people do, and so I've decided, prompted by fellows who also see music in shape and colour, to share this on my blogs. You see, when I wrote notes for music reviews, I have a tendency to put the colour down in my notes as well...I'll write something about the instruments, the vocals, the feel, and then "blue-green" or "glittery gold" or "pink and purple rain" or something equally apparently nonsensical! It never ends up in the music reviews, because that would just sound crazy :P
But I may post little notes about what else I've noted when doing music reviews and the like, just for fun, and if anyone else thinks that they may have synaesthesia and would like to join in, then go ahead! :)
In the meantime, if you're interested in synaesthesia, check out the wikipedia link:
(PS I loathe to call it a "condition" because that almost sounds like there's something wrong your brain. Its never done me any harm, although I will admit that it is difficult to perform mental maths when you're trying to add up coloured ribbons :P )