What is the center of an atom called?

Nucleus

What traditional Asian board game has players capture opponents' pieces using a river-divided board as an obstacle? Which sauce is commonly served with sweet-and-sour pork in Chinese-American cuisine? Which treaty divided Charlemagne's empire among his grandsons? In "Joyeux Noel," soldiers from how many nations agree on an unofficial Christmas truce? In what year did the Great Fire of London occur?