The Torah states in (Exodus 12:20), "All leavening you shall not eat." The Mekhilta asks why this verse is needed at all — since the Torah has already forbidden chametz in an earlier verse and already specified the punishment for eating it.
(Exodus 12:15) establishes the consequence: "Whoever eats chametz, that soul shall be cut off." This is kareth, the most severe spiritual penalty in the Torah. But here is the problem: kareth is a punishment, not a prohibition. Jewish law distinguishes sharply between the two. A punishment tells you what happens if you transgress. An exhortation — a direct "do not" command — tells you not to transgress in the first place. You need both.
The verse "All leavening you shall not eat" supplies the missing exhortation. It is the explicit prohibition that the punishment in the earlier verse presupposes. Without this "do not eat" command, the kareth penalty would technically have no formal prohibition to attach to — an anomaly in the Torah's legal structure.
But the Mekhilta presses further. The verse says "all leavening" — not just chametz, not just se'or (sourdough), but all leavening. The word "all" expands the prohibition beyond the primary substances to include their admixtures. If chametz has been mixed into another food — even if the resulting mixture is not itself pure chametz — the prohibition still applies.
A single word, "all," extends the scope of Passover law from obvious violations to subtle ones, from pure leavened bread to anything touched by its presence.