I always thought it's because they brush their teeth with rocks. I assumed everyone knew this.

Back in the heyday of online poker a few sites would have flag indicators of the players country of origin. If the chat box on the screen would get spicy with insults over a bad beat I would ask the above question... nothing triggers British people more than jokes about the condition of their fucked up looking teeth. Made for hilarious meltdowns in the chat. I know, I know, if you can't say anything nice... I couldn't help myself. :)