If you are Askenazi, you know that besides chometz, there is something else to be fearful to consume during Pesach: kitniyot. Practically, there is not an English translation of this term, but an approximation could be grains, with a Hebrew general translation as vegetables - which does not make any justice to the term, especially not from the religiously accepted point of view.
According to Rambam, on Pesach there is prohibited to consume the 5 kinds of grains: oats, rye, wheat, barley and spelt. Kitniyot are grains that are similar with the five grains - for instance, rice, millet, peas, lentils, but Rambam do not prohibit their use during Pesach. Therefore even flour ground from rice can be used to prepare different mixture.
The early source for the Askenazi minhag is apparently found at Rabbeinu Peretz (a Baalei Tosafot). According to him, the grains similar to those from the prohibited list, plus beans, if boiled operate similarly with the chometz (produced after more than 18 minutes of co-habitation between flour and water) are forbidden for Pesach.
For instance, in the case of beans, if you put them in the water and wait for them to boil, the process is problematic chometz-wise, the advice being to just throw them into the boiling water. The beans are a completely different category of discussion anyway. Rabbeinu Yechiel, for instance, also a Baalei Tosafot, is said to have been used to eat white beans on Pesach. However, the beans as a general category and specific examples are a very complicated issues, as many of them - for instance, soy beans - were introduced in the last century only, which opened the way to complicated discussion about their presence on the Pesach menu. The same applies to quinoa, which is a relatively new add to the culinary habits.
The minhag of refraining from consuming kitniyot, introduced by the Askenazi rabbis, was a gezeira (a decree) based on the fact that the kitniyot are prepared in the kitchen boiling them in a pot of water, therefore similarly with the chometz, therefore should not be consumed on Pesach. It is important to mention that this prohibition is not halachic, but a chumra, a strict standard adopted by the community. In addition to this, it also function the rule that if your parents had a certain Pesach eating custom, you can follow it as well. Last but not least, the minhag hamakom - the custom of the place - operates, meaning that if you are living in a community with stricter standards you should follow them. If the community is lenient and your standards are higher, you keep following your standards.
Rabbi Joseph Caro, the author of Shulchan Aruch, add another information regarding the reason of forbidding kitniyot: sometimes the five grains are mixed with lentils or other priducts, because the fields were near to each other and the wind may bring undeserved seeds. The reason is further outlined by the Taz - Rabbi Levi - in his commentary to Shulchan Aruch. The need to check for cross-contamination is required from the Sephardi communities consuming rice on Pesach. For instance, the rice is checked three times for chometz in a room where there are no children to the room, because their noise might distract from a serious search.
The discussion about kitniyot is never ending and there are many serious arguments pro and against them. More than any other holiday, on Pesach our many differences as communities and individuals are on display and the positive lesson we should learn is about being tolerant and accepting the food alterity of everyone. The care to respect the prohibition of eating chometz on Pesach is a valuable asset we should respect and transmit further to the next generation. Without hate or permanent complaining and disregard for other people's customs.
