Kitniyot: Not Quite Hametz
Although rice, millet, corn, and legumes are not among the five prohibited grains on Passover, Jews of European origin have traditionally avoided them during the holiday.
There are five grains, and five grains only, that, according to Jewish law, can ferment and become hametz. These are wheat, barley, spelt (also known as farro), oats, and rye. These are also the only grains that can be made into matzah. Traditional Jewish law forbids eating, owning, or deriving benefit from these five grains in any amount and in any form throughout the holiday (other than when they are baked into matzah).
Little Things That Make Big Problems
On its own, this rule requires fairly extensive effort to observe, but it is, at least, quite clear. For the past 700 years, however, Ashkenazic Jews have complicated Passover observance by avoiding rice, millet, and legumes. These are collectively known as kitniyot, from the Hebrew word katan (little).
Although the earliest mention of the custom to prohibit kitniyot dates from the 13th century, the discussion concerning their use goes back to Tannaitic times. The second century Rabbi Johanan ben Nuri considered rice and millet to be close enough to the five grains that one could use them for matzah:
"Our Mishnah [which defines the five grains that can leaven and can therefore be used for matzah] disagrees with R. Johanan b. Nuri, who holds: Rice is a species of grain, and one is punishable for [eating it in] its leavened state. For it was taught: R. Johanan b. Nuri prohibits rice and millet, because it is close to leaven" (Bavli Pesachim 35a).
Here, the Talmud points out that Johanan b. Nuri's approach disagrees with the Mishnah. A later Talmudic discussion mentions that the amoras (rabbis of the third to sixth centuries) R. Huna and Rava used to put rice on the seder plate, from which behavior, Rav Ashi concludes, "We do not pay attention to the opinion of R. Johanan b. Nuri" (Bavli Pesachim 114b).
Reasons for Prohibiting Kitniyot
During the 13th century, rabbis in France began to refer to a custom of prohibiting kitniyot, including rice, dried beans, millet, and lentils, although most of the reasons explaining (or justifying) the prohibition were developed later. The 13th century talmudist Rabbenu Peretz b. Elijah of Corbeil suggests that people might get confused because hametz and kitniyot are boiled similarly, and in some places they make kinds of "bread" out of kitniyot. People might wrongly assume that what is permitted for rice or beans might also be permitted for the five grains. In the 14th century, R. Jacob b. Asher, the author of the Arba'ah Turim, suggested that grain might be mixed up with kitniyot during storage.
Reasons for Rejecting the Custom
On the other hand, the custom has been widely criticized. As the Israeli Masorti (Conservative) Rabbi David Golinkin has shown, not only is the custom contrary to the opinions in the Talmud, but more than 50 different early sages reject it outright. One of the earliest sages to mention the practice, the 13th-century Samuel b. Solomon of Falaise, considered it a "mistaken custom," and others call it a "superfluous stricture" or even a "stupid custom."