What food item does a Christmas pudding often have hidden inside for good luck in British tradition?

A coin

What series features Katniss Everdeen as the protagonist? Who created the cartoon series "The Simpsons"? In wrestling, which medal was awarded to Rulon Gardner in the 2000 Olympics when he upset Alexander Karelin? Who sang the hit song "Royals" in 2013? Which treaty transferred the Philippines, Guam, and Puerto Rico to the U.S.?