Tomorrow begins the Jewish holiday of Passover, which means millions of people all over the world have furiously scrubbed their houses trying to rid them of every last trace of chametz, or leavening. Jews who observe the eight-day holiday will avoid eating anything leavened for the duration, and some will cook carefully prescribed Passover seders according to a strictly curtailed list of foods. What is and isn't OK to eat over Passover? Emily Yoffe explained the ins and outs in 2001. A hint: S'mores are not OK.