These techniques are often different in form than their counterparts in Western animation, and they include a fixed iconography that is used as shorthand for certain emotions and moods. For example, a male character may develop a nosebleed when aroused. A variety of visual symbols are employed, including sweat drops to depict nervousness, visible blushing for embarrassment, or glowing eyes for an intense glare. The opening and credits sequences of most anime television episodes are accompanied by Japanese pop or rock songs, often by reputed bands. They may be written with the series in mind, but are also aimed at the general music market, and therefore often allude only vaguely or not at all to the themes or plot of the series. Pop and rock songs are also sometimes used as incidental music ("insert songs") in an episode, often to highlight particularly important scenes. 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.