The Reaping Balls hold the slips of paper with the names of children from age 12 to age 18, to be possibly reaped in from each district. There are two Reaping Balls, one for boys and one for girls. During the reaping, the reaping balls are placed on a stage placed in front of the Justice Building for the district escort to choose a name from them.
During the 75th Hunger Games, the third Quarter Quell, the twist was that only past Hunger Games victors would be eligible to be reaped. In District 12, being that there were only three victors alive: Katniss, Peeta, and Haymitch Abernathy, Effie didn't have to claw around the girls' ball for a few minutes, as there was only a single slip of paper in it. She then chose Haymitch out of the boys' ball but Peeta immediately volunteered in his place.