Cricket for Aliens

Two aliens were visiting Earth to research the local customs. They split up so that they could learn more in the time allowed. When they met to share their knowledge, the first alien told of a religious ceremony it had seen. 'I went to a large green field shaped like a meteorite crater. Around the edges, several thousand worshipers gathered. Then two priests walk to the center of the field to a rectangular area and hammer six spears into the ground, three at each end. Then eleven more priests walk out, clad in white robes. Then two high priests wielding clubs walk to the center and one of the other priests starts throwing a red orb at the ones with the clubs.' 'Gee,' replied the other alien, 'what happens next?' 'Then it begins to rain.'