The ḥametz prohibited by the Torah is produced from one of the five types of grain: wheat, barley, spelt, oats, and rye. Other species such as rice and millet, even if they rise, do not undergo the same fermentation process as the five cereal grains, and they may be eaten on Pesaḥ. Although one Tanna, R. Yoḥanan b. Nuri, maintains that rice is also a cereal grain and forbidden by the Torah in its leavened state, the rest of the Sages maintain that even if rice rises, it may be eaten on Pesaḥ (Pesaḥim 35a). This was the practice of the great Tanna’im and Amora’im. In fact, Rava ate rice at the Seder (ibid. 114b).
During the era of the Rishonim, over 800 years ago, the Jews of the Rhineland would refrain from eating kitniyot on Pesaḥ. Initially, only some communities observed this stringency, but within a several generations the custom had spread to all Ashkenazic communities.
Three principal reasons for this custom have been offered: A) Since kitniyot are cooked in the same manner as grains, in a pot, there is concern that if people cook rice on Pesaḥ they will end up mistakenly cooking forbidden types of grain. B) Since kitniyot, like cereal grains, are often made into flour, if the unlearned masses see pious Jews cooking and baking foods with kitniyot flour without concern for it becoming ḥametz, they are liable to do the same with grain flour as well. The rabbis of the Talmud were not concerned about this because, in their day, Jewish tradition was clear and established. However, the tribulations of the exile and the scattering of Jewish communities gave rise to a fear that some Jews would be cut off from tradition and come to forget what is forbidden and what is permitted. Eating kitniyot on Pesaḥ would cause them to err and eat forbidden cereal grains without taking care that they do not become ḥametz. C) Grain and kitniyot kernels are similar in appearance and are kept in the same storehouses for long periods. It is therefore eminently possible that wheat or barley kernels would find their way into kitniyot, and when the kitniyot are cooked the grain will become ḥametz. This concern persists today, because the same storehouses are used for grain and kitniyot, and they are not usually cleaned out when switching from one species to another. Thus, in fact, when checking different types of kitniyot one can find kernels of grain. This is due to the fact that in order to replenish the fields and soil, crop rotation was instituted: the same field would be planted with grain one year and legumes (which help replenish the soil) the next year. However, plants of the previous crop inevitably remain in the field. Therefore, if a fenugreek crop was grown after a wheat crop, some wheat will sprout among the fenugreek, and some kernels of wheat will be found in the harvested crop of legumes. The same applies to all other species. Experience shows that sometimes the quantity of grain kernels is more than one sixtieth of the entire kitniyot crop. This problem applies to those species of kitniyot that physically resemble cereal grain.