The hymen or vaginal corona is a tissue, usually doughnut-shaped, inside the vagina. They don't "break" or bleed, but some of the hymen can stretch or tear. It is normal for some of the hymen to wear away due to normal activities such as bathing,
masturbation, even just moving around or stretching. Contrary to popular belief, it is actually pretty unlikely that a hymen will be damaged by
riding a bike or a horse, because all these activities really do is put pressure on the perineum, not interfere with the vagina itself. It does not completely cover the vagina; if it did,
menstruation would be impossible for girls with intact hymens. By the time full PVI occurs, the hymen is (usually) already mostly worn away.