In many households, part of the fun of eating Christmas pudding is finding a trinket that predicts your fortune for the coming year.  For instance, finding a coin means you will become wealthy.  Finding a button means that you will be:

A ring means you will get married; while a thimble predicts spinsterhood.  The idea of hiding something in the pudding comes from the tradition in the Middle Ages of hiding a bean in a cake that was served on Twelfth Night.  Whoever found the bean became "king" for the rest of the night.