What do we actually mean when we say chametz?
Any flour of the five species of grain, which is mixed with water and allowed to ferment before being baked, comes under the definition of chametz according to the Torah. The five species of grain are wheat, spelt, oats, barley, and rye.
The Sages established that the time necessary for the fermentation process to take place is eighteen minutes after water has been added to the flour; i.e., if water is added to the flour and nothing else was done to the mixture for eighteen minutes, the mixture becomes chametz.
The time when the mixture is being processed, kneading and rolling it out, is not part of these eighteen minutes. However, once the dough has been kneaded, we are careful not to leave it for a moment. By kneading it or otherwise working with it, the fermentation process is delayed and the mixture will not become chametz.
If the flour was mixed with only fruit juice or with eggs, it does not become chametz. However, because we fear that the water may inadvertently be mixed with the fruit juice or with the eggs, we do not do this; moreover, some authorities hold that fruit juice does ferment.
Chametz may not be consumed on Passover, either by eating it, or dissolving it in water and drinking it, and no benefit may be derived from it. It must be either burned or destroyed of in some manner.
Even a minute particle of chametz is forbidden on Passover. Even if the amount of chametz in a mixture is only 1/1000th of the total, the entire mixture is forbidden as chametz!
However, if chametz became mixed into food before Passover, it is nullified, provided that it is less than 1/60th of the volume of the entire mixture and that it has become completely dissolved into the mixture and cannot be discerned.
It is forbidden to eat chametz from midday on the fourteenth of Nissan, that is, from the beginning of the seventh hour. One who transgresses this prohibition is subject to lashes according to the Torah, for the verse states (Deuteronomy 16:3): You shall not eat chametz with it [the Paschal sacrifice]. The traditional explanation for this verse is that the prohibition of eating chametz starts from the time when the Passover sacrifice could be offered after midday of the fourteenth of Nissan.
The Sages, in order to prevent people from transgressing the prohibition inadvertently, decreed that the prohibition of eating and deriving benefit from chametz starts at the beginning of the sixth hour. Thus, during the sixth hour, the prohibition is Rabbinic; afterwards the prohibition is from the Torah.
One who willfully eats a piece of chametz which is as big as an olive, from the night of the fifteenth of Nissan until the end of the twenty first of Nissan incurs the penalty called karet, Divine excision, for the verse states: For whoever eats chametz, that soul shall be cut off from Israel (Exodus, 12:15).
One is permanently forbidden to derive any benefit from chametz which remained in one's possession during Passover. This prohibition is a penalty which the Sages levied to punish the person for having transgressed the Torah prohibitions of not seeing or having chametz in one's possession during Passover.
This penalty applies whether the chametz remained in his possession on purpose or by accident or through oversight. The Sages levied this penalty so that people would not leave chametz in their possession for use after Passover (ibid.).