The decision for who is the champion for each school is set upon it. It's kept in a ancient wooden chest which is encrusted with jewels. When the casket it tapped three times with a wand, it opens. It is then removed and set on top of the chest. It's a large, roughly hewn cup,  and filled to the brim with dancing blue flames. Anybody wishing to submit themselves as champion must write their name and school clearly upon a slip of parchment and drop it into the it. Aspiring champions have twenty-four hours in which to put their names forward.  The next night it will return the names of the three it has judged most worthy to represent their schools. It is placed in the entrance hall tonight, where it will be freely accessible to all those wishing to compete. To ensure that no underage student yields to temptation, Dumbledore drew an Age Line around the Goblet of Fire once it had been placed in the entrance hall. Nobody under the age of seventeen will be able to cross this line. It expels slip of paper for each chosen champion. But, in Harry Potter's fourth year it chooses four champions instead of three. Harry Potter himself being the forth champion. Barty Crouch, a Death Eater who was disguised as Moody, put his name in it. Once the names had been chosen, the Goblet's flames died out and would not reignite until the next Tournament.