The oracle sat with her back to the hill, a breeze riffling the ruby scarves tied to her folding camp chair. One by one, the 12 boys approached. They stood straight as the oracle lowered her sunglasses and looked them over. Sorting through a pile of paper slips with burned edges, the oracle, a middle-aged woman, selected one for each child. 'I will prophesize your quest,' she told Tom Leier, 9, before reciting a mysterious poem that would guide him for the week ahead. That morning, the boys had been regular Brooklyn elementary school students at a summer camp in Prospect Park. But now each had been revealed to be a half-blood, with one mortal parent and one who was a god of Greek myth. Children have always sought to act out elements of their favorite books, becoming part of the worlds that the works create. Now, organized role-playing literary camps, like the weeklong Camp Half-Blood in Brooklyn, are sprouting up around the nation. Some take their inspiration from the Harry Potter books, like the wizardry camp run by the Brandywine Learning Center in Chester Springs, Pa., which simulates the experience of attending Hogwarts, the school from the novels. Bookstores have joined in, organizing day camps structured around children's books like 'The Double-Daring Book for Girls' and the 'Ranger's Apprentice' series. But the biggest buzz has recently been around Camp Half-Blood, based on the popular 'Percy Jackson and the Olympians' series. In the Percy Jackson books, misunderstood children find out they are modern-day mythological heroes. Interest in Camp Half-Blood has been growing, perhaps because a Percy Jackson movie was released this year, or because the series features its own Camp Half-Blood, where Percy and other middle school demigods find refuge. An independent bookstore in Austin, Texas, held the first Camp Half-Blood in 2006. The store, BookPeople, had been hosting dramatic readings of manuscripts in the series, and one day Topher Bradfield, the children's activity coordinator, said to his young listeners, 'Wouldn't it be great if Camp Half-Blood was a real place?' 'The kids,' Bradfield recalled, 'looked at me as if I'd sprouted a second head, and were like, 'Yeah, duh. Of course!'' The day camp, which is held in a state park, attracts children from as far away as Brazil and England, who stay with their parents in nearby hotels. This year, the camp's 450 spots sold out in an hour and a half, Bradfield said. The camps run by bookstores, which are also in Decatur, Ga., and now in Brooklyn, are not fancy affairs. A casual observer of the various Camp Half-Bloods would see a few decorations and children in matching camp T-shirts jousting with foam swords or javelins. Gods, oracles and monsters are played by actors, counselors or volunteers. But the homemade nature of the experience, camp staff members said, permits students to create the illusion in their own minds. 'My biggest challenge has been getting parents to understand that we don't intend to sit indoors with their kids and read all summer,' said Crystal Bobb-Semple, the owner of Brownstone Books in Bedford-Stuyvesant, which started the Brooklyn camp this summer and charges $375 per week. 'It is experiential.' Camp Half-Blood in Brooklyn had 44 campers and a simple back story: Discord had come over Brooklyn. It was up to the campers, who would be pronounced the children of Apollo, Ares or Hermes, to find the five pieces of the Apple of Discord, a mythological object, to set things right. Not everything went according to plan. Prospect Park denied permission to put up a tent, so the camp's center consisted of three folding tables under tall trees. A nearby library and a movie theater were used in bad weather. As the second session of the camp opened, the dozen boys, age 7 to 11, squared off under a scorching sun for their first sword lesson, taught by a local Japanese sword-fighting instructor. There was some skepticism: When Nathan Mandell, 10, glimpsed the first piece of the Apple of Discord later that day, he did not buy it. 'That's a piece of foam with glitter on it,' he said. But the camp's director, Karenga Arifu, known as Achilles, referred to the boys as young heroes. They teamed for chariot races on the backs of pedicabs and corrected each other's mythology. 'There really are demigods, and I hope that's why I'm here,' said Tom, who wore a yellow bandanna to signify his possible Apollo parentage. After all, in the books, Percy Jackson does not find out that he is the son of Poseidon -- not just a struggling student -- until he is 12. 'I'm not here to pretend,' he said. 'I'm here to train.' (STORY CAN END HERE. OPTIONAL MATERIAL FOLLOWS.) Each day, three children were selected to go on a quest to defeat a monster and retrieve a piece of the magical apple. On Thursday, three children of Ares, the war god, set off into the wooded paths of Prospect Park. Their swords tucked into their shirts, they chatted with their counselor, Jason McConnell, 18, about the difference between medusas and gorgons. The trees closed in around them as they climbed a flight of stone steps and traversed a mound of wood shavings. Toilet paper dangled from the trees. They approached an overgrown circle of weeds, which Jason told them was the entrance to the Garden of Demeter. Up ahead there was a flash of color -- an orange camp T-shirt stained with fake blood. Then the monster -- a Fury -- jumped out of the bushes. He was obviously a teenager in an old-man Halloween mask with rubber hands -- right? The children began to fight him with their foam swords. The monster's sword struck Issa Chambers, 11, near the eye. He started to cry and retreated. 'Get back in there,' Jason told Issa, and he did, in a flurry of anger. He pummeled the monster with his sword until Nathan shouted, 'Issa, it's an actual person!' Walking back to camp, Issa said he felt bad for getting carried away. Jason reassured him. 'Ares gave you the rage,' he said. 