פירוש על פסחים 3:5

שיאור – that did not ferment all the way: [cracks] it is the manner of dough that rfermits that it makes many cracks.

Introduction This mishnah deals with how one can tell when a piece of dough has started to become chametz.

כקני חגבים – a crack to this side and a crack on the other side.

Si’ur must be burnt, while he who eats it is exempt; sidduk must be burnt, while he who eats it is liable to kareth. The halakhah in this section seems to be an old halakhah, one which Rabbi Judah and the other sages debate in the remainder of this mishnah. The halakhah is that “si’ur” dough must be burnt, meaning it must be removed before Pesah, but that one who eats it is not liable for karet as he would be were he to have eaten chametz. “Si’ur” comes from the word for hair (se’ar) and we will see in the remainder of the mishnah that both Rabbi Judah and the sages explain it so that it has some connection with hair. “Sidduk” means “cracked” and it is already chametz and therefore one who eats it is liable for karet.

זה וזה האוכלו חייב כרת – for the locust’s horns also are a crack. And what is a fermentation that would be exempt? This which has no crack, but if the dough became pale and wrinkled like a person whose stair stood up out of fright [that his face grows pale]. And the Halakha is according to the Sages.

What is si'ur? [When there are lines on the surface] like locusts’ horns. Sidduk is when the cracks have intermingled with each other, the words of Rabbi Judah. Rabbi Judah and the sages now debate the definition of si’ur and sidduk. According to Rabbi Judah si’ur dough already shows some signs of cracking on the surface and the lines look like locusts’ horns. These horns are similar enough to hair that the dough is called si’ur. The lines on sidduk dough are more numerous and have already begun to intermingle with each other. One who eats dough with this many cracks is liable for karet for having eaten chametz.

But the sages say: regarding the one and the other, he who eats it is liable for karet. And what is si'ur? When its surface is blanched, like [the face of] a man whose hair is standing [on end]. The sages say that what Rabbi Judah called si’ur is actually sidduk and therefore one who eats it is liable for karet. They redefine si’ur as dough that has begun to turn white, like the face of a person who is frightened. Note again the connection with hair to the sages this dough is called si’ur because when a person is frightened not only does his face turn white but his hair stands on end.