at the general music market, therefore they often allude only vaguely or not at all, to the thematic settings or plot of the series. Also, they are often used as incidental music ("insert songs") in an episode, in order to highlight particularly important scenes. Genres Anime are often classified by target demographic, including , , and a diverse range of genres targeting an adult audience. Shoujo and shounen anime sometimes contain elements popular with children of both sexes in an attempt to gain crossover appeal. Adult anime may feature a slower pace or greater plot complexity that younger audiences may typically find unappealing, as well as adult themes and situations. A subset of adult anime works featuring pornographic elements are labeled "R18" in Japan, and are internationally known as hentai (originating from ). By contrast, some anime subgenres incorporate ecchi, sexual themes or undertones without depictions of sexual intercourse, as typified in the comedic or harem genres; due to its popularity among adolescent and adult anime enthusiasts, the inclusion of such elements is considered a form of fan service. Some genres explore homosexual romances, such as yaoi (male homosexuality) and yuri (female homosexuality). While often used in a pornographic context, the terms yaoi and yuri can also be used broadly in a wider context to describe or focus on the themes or the development of the relationships themselves. Anime's genre classification differs from other types of animation and does not lend itself to simple classification. Gilles Poitras compared the labeling Gundam 0080 and its complex depiction of war as a "giant robot" anime akin to simply labeling War and Peace a "war novel". Science fiction is a major anime genre and includes important historical works like Tezuka's Astro Boy and Yokoyama's Tetsujin 28-go. A major subgenre of science fiction is mecha, with the Gundam metaseries being iconic. The diverse fantasy genre includes works based on Asian and Western traditions and folklore; examples include the Japanese feudal fairytale InuYasha, and the depiction of Scandinavian goddesses who move to Japan to maintain a computer called Yggdrasil in Ah! My Goddess. Genre crossing in anime is also prevalent, such as the blend of fantasy and comedy in Dragon Half, and the incorporation of slapstick humor in the crime anime film Castle of Cagliostro. Other subgenres found in anime include magical girl, harem, sports, martial arts, literary adaptations, medievalism, and war. Formats Early anime