To operate, grails must be placed onto large, mushroom-shaped "grailstones", found at intervals along the riverbanks, which produce an electrical discharge three times per day (corresponding to the times of breakfast, lunch and dinner). As agriculture is absent and impossible on the Riverworld, the grails are vital to an individual's survival, and cannot be opened except by their individual owners. Nevertheless, "grail slavery" is not uncommon, in which a person is held captive and the contents of his or her grail, retrieved by the owner, are taken by force by the captor. The slaver will usually provide the slave with enough food to keep him alive, as once a person dies their grail becomes useless. Of special value are "free grails", originally found atop each of the grailstones as a demonstration of their relationship. Free grails can be opened by any individual, and as such, are valued for an extra ration of goods at each interval. Throughout the series, several main characters lose their original grails and must seek free grails to survive. Though the grails provide for all needs and the climate is hospitable, further attempts to affect the environment are frustrated by the near-complete lack of metals and ores on the planet. The only building materials available are bamboo, wood, and human or fish bones and hides. Pockets of flint (eventually depleted) provide material for tools. With technology limited to the paleolithic level, the surrounding mountains are impassable. Travel along the river is hindered by division of the Riverworld into thousands of empires, monarchies, republics, and other social systems, each only a few kilometers long and housing 90 people per square kilometer. Because the distribution of populations along the river seems random, the character of these nations can vary wildly within a very short span; one may enter dangerously unknown and potentially hostile territory in less than a day's journey. The reason of the existence of Riverworld is initially a complete mystery. In Farmer's books a number of historical figures — including Sir Richard Burton, Alice Hargreaves, Samuel Clemens, Baron de Marbot, King John of England, Cyrano de Bergerac, Tom Mix, Mozart, Jack London, Lothar von Richthofen, and Hermann Göring — interact with fictional characters to discover the Riverworld's purpose. Another character, Peter Jairus Frigate, bears a striking resemblance to Farmer himself, and shares his initials. There are two versions of the character: one who appears early in the sequence, the third overall character to be introduced, and another, the "real" version, who concludes that the first was his brother who died as a baby, resurrected and used as a spy by the creators of the Riverworld.