The Torah commands in (Exodus 12:17), "And you shall watch over the matzot." The Mekhilta takes this verse as the foundation for one of the most detailed areas of Passover law: the precise rules governing when dough crosses the line from matzah into chametz.
"Watch over the matzot" means ensuring they do not become unfit — that the dough does not begin to ferment and rise. From this single phrase, the rabbis derived an entire set of practical rulings for the kitchen.
If a woman is kneading dough and notices it starting to rise, she should immediately douse her hands in cold water and pull the dough apart. Cold water halts the fermentation process, buying precious time before the dough becomes irreversibly leavened.
But if the dough has already become se'or — a fully leavened sourdough — the situation changes entirely. It must be burned, and anyone who eats it is liable to kareth, the severe spiritual penalty of being "cut off" from Israel.
Rabbi Yehudah then defines exactly what se'or looks like. The dough has reached the point of no return when it develops "locust horns" — small protrusions on its surface that resemble the antennae of a locust. Another sign is sidduk, or "splitting," when cracks appear on the dough's surface and begin to intersect with each other. Once either sign appears, the dough is chametz and must be destroyed.
These vivid physical descriptions transformed abstract law into something any baker could recognize at a glance.