Synesthesia is what happens when things that are supposed to press one sensory button accidentally brush against other ones on their way through. It can happen in all sorts of combinations, but the most common forms are where numbers or letters appear to have colours or sounds.
Because my brain doesn't do "common", I have a much weirder variant of synesthesia: ordinal linguistic personification. It's hilarious: for me, numbers have personalities. It affects some words and objects as well, but it's most obvious - and ridiculous - with numbers.
I perceive most odd numbers as male and evens as female, something my brain decided for itself without Pythagoras' help. But some, especially once you get into higher numbers, have indistinct or fluid gender. Some are young and some are old, and that bears no relation to their numerical value. Some are fairly bland - 1 is just a guy, standing there - while others have fully formed personalities.
Mathematics usually doesn't come into it - the personality of 3 is in no way a combination of the characters of 1 and 2. But then sometimes it does, because 48 is 6x8 and that's just skeevy and wrong and possibly incestuous. But 21 is 3x7 and that's awesome. I don't know why. Ask my brain.
Speaking of brains, we're not altogether sure what causes this particular form of synesthesia. One theory is that there are some crossed lines between the bit of the brain that recognises sequences of numbers, and the nearby bit that does theory of mind and picking up peoples' personality traits. Another school of thought locates the quirk in the magnificently named retrosplenial cortex,which sounds like it should be a Gothic cathedral with dark wood and a high vaulted ceiling, with light streaming in through coloured glass.
I'm not particularly good at mathematics. Once it comes to theoretical stuff with x's in it, I'm really quite rubbish. I'm utterly inept at number puzzles like sudoku as well. Whether or not this is related to how I perceive numbers, I have absolutely no idea.
So, what are the personalities of these numbers like? 1 is fairly nondescript. 2 is plump, friendly and hard working, whereas 4 is slimmer and takes no shit. 5 is earnest and serious and hands-on, unlike 3 who's more inclined to academic, philosophical questions. 6 and 8 are both very feminine, although 6 is giggly and girlie and 8 is more grown up and sophisticated. 7 is that guy you don't trust but you're not sure why, and 9 is a wise old bloke with a beard.
Here's a chart, to help put that in pictures: