The Torah says the Passover lamb must not be "cooked in water" (Exodus 12:9). Water is specified. But Rabbi Yishmael immediately sees the problem: what about wine? What about fruit juice, oil, or any other cooking liquid? If only water is mentioned, does that mean boiling the lamb in wine is permitted?
The Mekhilta records Rabbi Yishmael's elegant solution. He constructs what the rabbis call a kal va'chomer — an a fortiori argument, a logical inference from lesser to greater. Water, he observes, does not impart its own taste to the food cooked in it. When you boil meat in water, the meat tastes like meat. The water is a neutral medium. Yet the Torah still forbids cooking the Pesach (Passover) in water.
Other liquids — wine, oil, vinegar, fruit juice — do impart their taste. They change the flavor of whatever is cooked in them. If cooking in a tasteless liquid is forbidden, then cooking in a flavorful liquid must certainly be forbidden. The lesser case (water) establishes the rule; the greater cases (all other liquids) follow automatically.
This is one of the foundational methods of rabbinic reasoning, and Rabbi Yishmael was its greatest champion. He identified thirteen principles by which the Torah is interpreted, and the kal va'chomer stands first among them. The Torah did not need to list every possible liquid. It mentioned water — the mildest, most neutral option — and trusted human logic to extend the prohibition to everything more potent. God, in Rabbi Yishmael's view, speaks to intelligent readers. He gives them a principle and expects them to run with it.