A baker's dozen is thirteen instead of twelve. When you order a dozen donuts and the baker gives you thirteen, that extra one makes it a baker's dozen.

This unusual counting started centuries ago in England, where bakers could be fined or punished for selling underweight bread. Since loaves don't always come out exactly the same size, bakers started throwing in an extra roll or loaf when selling by the dozen. That thirteenth item guaranteed they wouldn't accidentally shortchange their customers, even if one piece was slightly small.

Today, people use the phrase to mean thirteen of anything, not just baked goods. You might say you have a baker's dozen of baseball cards or pencils. Some businesses still follow the tradition: a bagel shop might give you thirteen bagels when you order a dozen, or a flower shop might add an extra bloom to a dozen roses.