Have you heard of synesthesia? Wikipedia has.
My sister was home for the weekend. She's in the middle of a psychology degree, and I always like to chat with her about interesting things we've learnt. She was particularly interested in some of my books, like "Did You Spot The Gorilla?" by Richard Wiseman, amongst others.
This time, we got talking about synesthesia. I'd read briefly about this before - where people ascribe personalities to numbers of letters, or associate certain colours with letters, for example. I think I learnt about it from a documentary about a guy with Asperger's Syndrome ((Ass-burgers)) - ...