Every comic fans has their own head canon. Some are big additions. Others are small. Frankly, at this point, the official "canon" of the DCU seems to be whatever head canon the fans want to use, so I figure let's go wild.
I'll start with one I heard from Robanker that I love. Clark Kent hates wizards. He just hates magic. Even when he was a kid watching a magic show at some birthday party in Smallville, he was the kid at the back giving the dude pulling a rabbit out of his hat the stink-eye. That's just perfect.
What about you guys? What's head canon do you love?