Fertilization is the process by which a sperm combines with an oocyte, or egg cell, to produce a fertilized zygote. The sperm released during ejaculation must first swim through the vagina and uterus and into the fallopian tubes where they may find an oocyte. After encountering the oocyte, sperm next have to penetrate the outer corona radiata and zona pellucida layers of the oocyte. Sperm contain enzymes in the acrosome region of the head that allow them to penetrate these layers. After penetrating the interior of the oocyte, the nuclei of these haploid cells fuse to form a diploid cell known as a zygote. The zygote cell begins cell division to form an embryo.
Apart from the man putting his penis into the woman's vagina, the sex organs can be used in other ways in sexual intercourse. The man and woman can rub each other's sex organs with their hands. They can use sex toys such as dildos or vibrators . They can also engage in oral sex . When a person uses his or her mouth, lips and tongue to touch a woman's clitoris and vulva, this is called cunnilingus . When a person uses his or her mouth, lips, and/or tongue to touch a man's penis, this is called fellatio. A man or a woman (using an strap-on dildo or harness) can also put the sexual organ into a person's anus , this is called anal sex .