Why is a baker's dozen 13?
The Real Answer
A baker's dozen means 13, but the exact origin of the custom is not completely certain. Historical sources connect the expression to an old practice of bakers supplying an extra loaf when selling a dozen, and later explanations tie that generosity to bread-weight rules and the risk of selling short weight. What is not well supported is the dramatic story that bakers routinely added the thirteenth loaf to avoid having a hand chopped off.
You hear 'baker's dozen' and accept thirteen immediately, but add one story about a baker losing a hand and that becomes the only part you remember. The next time someone asks why it means thirteen, the punishment story is the first thing you're tempted to repeat.
The Dunce Dynamics Institute Examines Why People Ask This
Thirteen is such a strange exception to the word 'dozen' that people naturally want an origin story. The extra-loaf custom is historical; the severed-hand version is the memorable part that outran the evidence.