Clothed female, naked male is a genre of erotica featuring one or more nude men and one or more clothed women. A facial is a form of non-penetrative sex, though it is generally performed after some other means of sexual stimulation, such as vaginal sex, anal sex, oral sex or masturbation. The interaction between tops and bottoms—where physical or mental control of the bottom is surrendered to the top—is sometimes known as power exchange, whether in the context of an encounter or a relationship. Men and women may masturbate nude milfs until they are close to orgasm, stop for a while to reduce excitement, and then resume masturbating. The effect is to give the viewer the sense that they are experiencing the sex acts that they are watching, as opposed to simply watching others as a voyeur. Masturbation has been depicted in art since prehistoric times and is mentioned and discussed in very early writings. Fisting may be performed with or without a partner. Guro or Gore is a genre of Japanese pornography involving blood, gore, disfiguration, violence, mutilation, urine, enemas, or feces. There are currently no known laws forbidding the crushing of objects and insects, however the production or trade of crush erotica involving live vertebrates is condemned by animal rights activists and is illegal in many countries, including the United States and Great Britain. Teacher and Student or Boss and Secretary pornography involves gay sex video sexualized situations which take place between an authority figure, such as a teacher or 3d hentai boss, and an underling, such as a student or secretary, often fake. Some couples use a threesome as a way to develop a love triangle . The buttocks in human females thus contain more adipose tissue than in males, especially after puberty. Gang bang pornography is a type of pornography that depicts an individual engaging in physical sexual activity with multiple partners sequentially or at once. In pornography, depictions of lesbian sex form a popular subgenre, directed toward a male heterosexual and female homosexual audience.