The Hymen.

Taken directly from: http://www.the-clitoris.com/n_html/hymen.htm (Photos are not safe for home/school)

It appears that many if not most people are under the impression that the hymen is located within the vagina. It is, as the photographs and illustrations on this (linked) page reveal, part of the vulva, is external to the genital organs. It is located outside the vagina. The hymen is a layer of tissue that partially conceals the vaginal orifice of some girls and women. The hymen is also referred to as a girl's "cherry" or maidenhead.

During the early stages of fetal development there is no opening into the vagina. The layer of tissue that conceals the vagina at this time usually divides incompletely prior to birth. The size and shape of this opening or openings varies greatly from one girl to the next. There are girls who do not have a hymen at birth, as the tissue divides completely while they are still in the womb. Sometimes, this formation of an opening does not occur, resulting in an imperforated hymen. A doctor should examine an infant girl's vulva soon after birth to ensure her hymen is not imperforated, as should a girl's parents. (If menses is not permitted to flow freely from the body, extreme pain and cramping can result during menarche; a girl's first menstrual period.)

The tissues of the vulva are generally very thin and delicate prior to puberty. Any activity that places tension on the vulva tissues may stretch or tear the hymen. As a result, many girls and teens tear or otherwise dilate their hymen while engaging in physical activities such sports, horseback riding, inserting and removing tampons, and while masturbating. A girl may not know this has occurred, since there may be little or no blood loss or pain experienced during this event. It may also occur when she is too young to remember or understand what has occurred.

The presence or absence of a hymen in no way indicates a girl's virginal state. No one can determine by physical examination alone whether a woman or teen has engaged in vaginal intercourse. Only about 50% of teens and women experience bleeding the first time they have intercourse, so blood stained bed sheets are not a reliable indicator of prior virginity. The hymen of some women tear on more than one occasion. There are even hymen that are elastic enough to permit a penis to enter without tearing, or tear only partially. This is usually true only if the dilation first occurs very gradually with fingers or other objects over an extended period of time. Virginity is a spiritual attribute, not a physical one.

The hymen does not magically disappear when something is inserted into the vagina, it will only stretch or tear sufficiently to permit entry of whatever is being inserted. If for example, a teen inserts two fingers into her vagina while masturbating, her hymen may still tear when she has vaginal intercourse for the first time, since the average penis is larger than her two fingers. A woman who has had vaginal intercourse may still have hymeneal tissue present; this remaining tissue can be the cause of pain during intercourse. If a woman's current partner has a larger penis than her prior partners, or a couple tries a new technique or position during intercourse, her hymen may tear again, or for the first time. When doctors examine preadolescent and adolescent girls for evidence of sexual abuse, they look for injuries to the hymen; the hymen may still be intact except for a single tear. Remnants of the hymen are usually present until a woman delivers a baby vaginally.