Rabbi Yishmael cuts through the debate about burning Passover leftovers with a characteristically logical argument. The other sages needed the repeated phrase "until morning" to establish that leftovers cannot be burned on the festival day itself. Rabbi Yishmael says the verse is unnecessary for that purpose — the conclusion follows from a more basic principle.
The Torah states, "No labor shall be done in them" (Exodus 12:16) — referring to the festival days. Burning is categorized as labor. Therefore, burning Passover leftovers on the festival is automatically prohibited, with or without the extra verse. You do not need a special scriptural derivation to reach that conclusion. The general prohibition against work on holidays already covers it.
But if the verse is not needed for its apparent purpose, then why is "until morning" repeated? Rabbi Yishmael redirects the extra phrase to solve a different problem entirely: what happens when the eve of the sixteenth of Nissan falls on Shabbat (the Sabbath)? Normally, the leftovers would be burned on the eve of the sixteenth. But if that day is Shabbat, burning is prohibited. So the destruction of the leftovers is pushed to the seventeenth.
The elegance of Rabbi Yishmael's reasoning is in its economy. He refuses to use a verse for something that logic alone can establish. Verses are precious — they must be reserved for laws that cannot be derived any other way. This principle, that Scripture never wastes words on what can be reasoned independently, is one of the hallmarks of Rabbi Yishmael's school and one of the most influential ideas in the history of Jewish legal interpretation.