Pesachim 35 - What to make matzah from?
One can only make matzah from wheat, barley, spelt, rye and oats. Why only these? - Because the Torah said, "With the Passover offering you don’t eat any leaven… for seven days you will eat matzah." Thus, only things that can become leaven can serve for matzah for the Passover seder, and not, for example, rice.
Can you revert this rule and say that only things that can be made into seder matzah are prohibited as hametz? Not quite, and here is why. Rav Pappa and Rav Huna were sitting in front of Rav Idi, who was dozing off. They said between themselves, "Why is it that Resh Lakish says that if one makes dough with fruit juice and it leavens, one is still not liable on Passover? - It must be because this matzah is too rich, and one needs to remember affliction and eat matzah of poverty. But, on the other hand, if one dissolves flour in water and it leavens, he is liable, even though it is not fit as matzah for the seder!?" At this Rav Idi woke up and said, "Kids! The reason of Resh Lakish is different: dough made with fruit juice does not leaven!"
One also fulfills his obligation to eat matzah if it is made from flower that has not been fully tithed, and has minor defects in the tithing procedure, but not major ones.
Art: St. Peter Sleeping by Giuseppe Antonio Petrini