Why is a baker's dozen thirteen?

Why is a baker's dozen thirteen?

ANS: It's from an old tradition that the baker would want to test (taste) one of whatever he baked. There may be more to it.

ANS: The term arose because a law was passed in England in 1266 specifying exactly how much a loaf of bread should weigh, and it imposed a heavy penalty for underweight loaves. Bakers took to giving their customers an extra, or thirteenth, loaf so that they would be assured of meeting the weight minimum imposed by law. The term baker's dozen came about in the 16th century.