The Mekhilta continues its relentless cross-examination of Rabbi Yehudah's position that chametz must be destroyed specifically by burning. A new argument emerges — and a new counterexample tears it down.
The argument runs like this: both nothar (leftover portions of the Paschal lamb) and chametz carry the punishment of kareth — being spiritually "cut off" from the community of Israel. Since we know nothar must be burned, and since chametz shares the same severe penalty, perhaps chametz must also be burned.
The severity of the shared punishment seems to clinch the case. But the Mekhilta immediately produces a devastating counterexample: the fat of a stoned ox. If an ox gores and kills a person, the animal is stoned to death, and it is forbidden to derive any benefit from it — including eating its fat. The penalty for violating this prohibition is also kareth. Yet no one argues that the stoned ox must be burned. It simply must not be used.
The counterexample demolishes the logic. If kareth alone were sufficient to require burning, then the stoned ox would need burning too. Since it does not, the shared punishment of kareth cannot be the reason chametz requires fire.
This back-and-forth — building an argument, then dismantling it with a single countercase — is the hallmark of rabbinic legal reasoning. Every principle must survive the toughest possible challenge before it becomes law.