The Torah (Shemot 13) tells us regarding the holiday of Pesach: "Matzot shall be eaten for seven days; neither leaven nor sourdough shall be seen in all of your borders." The leaven that the Torah prohibits is produced by the combination of grain-flour and water remaining in this state for a long enough period of time; this causes the internal composition of the flour to leaven (rise). From the moment the mixture begins to leaven it is considered "Chametz" which is prohibited for consumption or benefit on Pesach and it is also prohibited for a Jew to retain Chametz in his property on Pesach.

 

Rice and all other legumes, including peas and beans, are permitted on Pesach, for the prohibition of Chametz only applies to grains and legumes are not classified as "grains". One must nevertheless take care to check the grains of rice well to make sure that no kernels of wheat or barley are mixed inside, for it is fairly common that in some places where rice is grown or packaged, other grains are also grown or packaged in close proximity as well and some kernels of these grains can easily become mixed into the rice and indeed make one's entire dish prohibited for consumption (for even one tiny crumb of Chametz can prohibit a large pot of food). Thus, it is customary to check through rice three times before Pesach with all due concentration and seriousness, at a time when small children are not present to disrupt the individual checking.