Some people like to hide a coin or trinket in the Christmas pudding.  This may have originated in the ancient custom, in Rome and elsewhere, of concealing a particular object in food.  The object was:

During the Roman festival of Saturnalia, a dried bean would be hidden in the food.  Whoever found it was then "master of the revels" - a king for the holidays.  Even a slave could be the lucky one.  In medieval times, a cake was eaten on Twelfth Night (January 6), during the most boisterous party of the year.  The "King of the Bean" ruled the whole party.