Fun fact time:
Did you know that a baker's dozen is 13? Ever wonder why?
Well as the story goes, they started putting a heavier tax on baked goods that did not weigh a predetermined amount, for example if the loaf of bread was to way 4 grams, and they provided something that only weighed 3 grams, they would have to pay a fine.
So, to avoid paying the fine, bakers began to bake by the "baker's dozen" and make an extra muffin, loaf, what have you.
Kinda cool huh?
Source: a really cool book my grandma got me when I was 10. Don't quote me on this, it's just a kid's book.