The traditional Mayas generally assume the Moon to be female, and the Moon's perceived phases are accordingly conceived as the stages of a woman's life. The Maya moon goddess wields great influence in many areas. Being in the image of a woman, she is associated with sexuality and procreation, fertility and growth, not only of human beings, but also of the vegetation and the crops. Since growth can also cause all sorts of ailments, the moon goddess is also a goddess of disease. Everywhere in Mesoamerica, including the Mayan area, she is specifically associated with water, be it wells, rainfall, or the rainy season. In the codices, she has a terrestrial counterpart in goddess I. The sources for Maya lunar mythology are almost entirely contemporaneous, with the exception of the Popol Vuh. A division can be made according to the moon's kinship roles. Moon as a male sibling: celestial power. In the Popol Vuh (16th century), the Maya Hero Twins are finally transformed into sun and moon, implying the recognition of a male moon, in a departure from the main Maya tradition. However, the Popol Vuh hardly belongs to lunar mythology, and becoming Sun and Moon may well be a metonym for acquiring dominance over the sky and thus, metaphorically, political predominance. Moon as a wife: origin of menstruation. True lunar mythology is first and foremost represented by the Qʼeqchiʼ myth of Sun and Moon first studied by Eric Thompson. It makes the Moon Goddess (Po) the daughter of the Earth God, or 'Mountain-Valley'. She is wooed and finally captured by Sun. They sleep together. When this is discovered and the couple flees, the angry father reacts by having his daughter destroyed. In all likelihood, this patriarchal punishment of a basic infraction of the rules of alliance represents the origin of menstruation, the 'evil blood' of a disobedient daughter colouring the water of sea and lake red, or sinking into the earth. The menstrual blood is stored in thirteen jars.