Faithful Muslims around the world begin their annual month-long Ramadan fast next week, during which they vow to refrain from all food, liquids, cigarettes and sex during daylight hours for the entire month.
According to the Koran, the fast is to begin at the moment of daybreak when it is possible to tell a black thread from a white one, and end only when the fading light of dusk makes that distinction impossible.
For practical purposes, Ramadan is scheduled to begin on Monday, but technically the fast does not begin in a particular area until local Muslim authorities have caught sight of the new moon.
Because the Muslim calender is 11 days shorter than the solar calendar, the month of fasting comes earlier each year. The day-long abstention from liquids works special hardship on the faithful when it occurs, as it has in the past few years, during the hot summer months.
A complex set of rules provides some conditional, and some full, exemptions from the fast.
Travelers, those who are ill, pregnant women and mothers who are breast-feeding may be excused, but they are expected to make up the days missed after the circumstances of the exemption are removed.
Elderly or incurably ill persons may be excused from fasting, but are admonished instead to contribute to the poor the equivalent of a day's rations for each fast day.
A person who inadvertently breaks most of the rules of the fast is expected to make up for it by observing an extra fast day after Ramadan. But one who breaks the restriction against sexual intercourse must atone by fasting for 60 days or feed 60 poor persons.
Using eye-drops, receiving medical injections, applying perfume or cosmetics and rinsing the mouth with water -- as long as it is not swallowed -- are all permitted.