Yes, yes, I know the rules for using apostrophes are simple - apart from the rule for it's/its, which you can understand from a pragmatic point of view - but why is won't not spelt wo'n't?
Just tell me that if you're so clever!
Word To Use Today: won't. This word is, obviously, short for will not.
I suspect that ca'n't should really have two apostrophes, too - though I sha'n't be starting a campaign to get them introduced.