Pesachim Chapter 3, Mishnah 5. This Mishnah describes the two distinct stages in the process by which dough becomes chametz, and each of them carries a different ruling regarding the severity of the punishment involved.
The two stages in the Mishnah:
"Si'ur" - a state of leavening found at the very beginning of the process. Dough that has reached this stage must be burned, but one who eats it is exempt from kareis and does not even receive lashes, since at this stage it falls into the category of chametz nuksheh - chametz that is not fit for eating.
"Siduk" - when the cracks and fissures in the dough have multiplied as a result of the fermentation. This too must be burned before Pesach, and one who eats it is liable to kareis, since this is full-fledged, complete chametz.
Defining the stages according to Rabbi Yehudah:
Si'ur: when very fine cracks appear in the dough, like "the antennae of locusts," running out in different directions.
Siduk: when those cracks begin to intertwine and join one another - a more advanced stage, which is considered full-fledged chametz.
The view of the Sages:
The Sages disagree and hold that both states - both the fine lines of the cracks and the cracks that join one another - are considered full-fledged chametz, and both are included in "siduk," the later stage. The reason, as the Gemara explains: even though from above it appears to have only a few cracks, underneath the cracks have already multiplied greatly.
If so, what is "si'ur" according to the Sages, that lighter level which is not full-fledged chametz?
This is dough whose surface has turned pale, like a person whose hair stands on end out of fear and whose face turns white. At this level the dough is considered chametz only partially, and one who eats it is not liable to kareis or to lashes; nevertheless, there is an obligation to burn it.