The definition clash also appears with the English definition of yuri as any lesbian relationship, as opposed to its sexually explicit definition in Japan. Sexologist Alfred Kind suggested that the buttocks is the primary sexual presentation site in primates. Sub-genres can also be classified into the characteristics of the performers or the type of sexual activity on which it concentrates, and not necessarily on the market to which each sub-genre appeals. The full-weight body-pressure, hentai porn moisture, sex odors and darkness can be perceived as powerful sexual attractions or compulsions. The Silent Duck, also called Duck-Billing, is the technique often used in which the person engaging in hand insertion shapes the hand to resemble a duck beak. Voyeur pornography may also include shots of topless women on topless beaches who are unaware of the cameras but may legally be photographed due to the public setting. Omorashi: The act of holding one own urine until the need to urinate is urgent, making another hold in their urine, or watching another person with an urgent need to urinate. Such ejaculation can be done by mammary intercourse, or masturbation over the other person, or result from the withdrawal of the penis from a partner mouth as he is ejaculating following fellatio, irrumatio, or intercourse. Pro-am pornography is sub-genre of amateur pornography that refers to professionally made porn that features amateur actors/actresses, often appearing for the first time in a pornographic film. Both sexes sometimes apply lubricating substances to intensify sensation. Compensating elements of the porn milf total dominance and submission are care and devotion complementing one another, thus facilitating stable relationships. The most common type of erotic spanking is administered videos porno gay on the bare buttocks, but can also be combined with bondage, in order to heighten sexual arousal and feelings of helplessness. Films with risqué content commenced to be produced soon after the invention of the motion picture in the 1880s.