Meilah, Chapter 3, Mishnah 3. This chapter continues to map out the moments at which consecrated items enter into the laws of meilah and the moments at which they leave them. Here the Mishnah takes up two items that are poured rather than eaten or offered up as flesh: the blood of korbanot and the nesachim, the wine or water libations poured onto the mizbe'ach.
Background: nesachim and the shitin
Wine libations accompany certain korbanot, and wine may also be donated on its own as a voluntary offering. Water is poured on the mizbe'ach as the water libation during the days of Sukkot. Practically, the liquid is poured into a silver basin set at the top of the mizbe'ach, flows out through a spout, and travels down into a cavity beneath the mizbe'ach known as the shitin.
The Mishnah's principle
Rabbi Yishmael frames the halachah as a pair of opposites: "Hadam kal bit'chilato vechamur besofo, vehanesachim chomer bit'chilatan vekal besofan." As far as meilah is concerned, blood starts out in leniency and finishes in stringency: at first no liability attaches to it, and only afterward does liability arise. The nesachim travel the reverse route: liability applies while they are still on hand, and it disappears once they have been used.
Blood: exempt at first, liable later
"Hadam bit'chila ein mo'alin bo." From the moment the blood leaves the neck of the animal at the time of shechitah until the zerikah, the throwing of the blood on the mizbe'ach, there is no meilah with it. This exemption is derived from a pasuk.
"Yatza lenachal Kidron, mo'alin bo." After the zerikah, the remaining blood drains across the floor of the courtyard and out of the Beit Hamikdash, flowing down into the Kidron Valley. Once it has reached there, one who derives benefit from it has committed meilah.
This liability, however, is rabbinic rather than biblical. On a Torah level, once a consecrated item has served the mitzvah for which it was designated, meilah no longer applies to it, and the blood has already served its purpose in the zerikah. The concern was practical. The blood in the Kidron was accessible to anyone, and people might well have taken it for fertilizer or similar uses. The Sages feared that such casual use would blunt people's sensitivity and lead them eventually to misuse consecrated items whose mitzvah had not yet been performed. They therefore decreed that meilah applies to this blood, and that a person must redeem it before deriving benefit from it.
That enactment, however, was deliberately limited. While the blood is still running along the floor of the azarah on its way outward, the Sages left matters as the Torah left them, since they were unwilling to pile extra restrictions onto life within the Beit Hamikdash. The upshot is precisely what Rabbi Yishmael formulated: with blood, the severity arrives at the finish and not at the start, since liability takes hold only after the blood has passed beyond the Mikdash precincts.
Nesachim: liable at first, exempt later
"Hanesachim bit'chila mo'alin bahen." So long as the wine or water has not yet been poured upon the mizbe'ach, one who benefits from it is liable for meilah. Nesachim carry the sanctity of kodshei kodashim, and liability for meilah attaches to them as soon as they are sanctified.
"Yatz'u lashitin, ein mo'alin bahen." With the liquid's descent into the shitin below the mizbe'ach, liability for meilah comes to an end. The pouring has discharged the mitzvah, and where the mitzvah has been fulfilled the liability lapses. Hence someone who collected the poured wine in a keli while it was still on its way down, before it reached the bottom of the shitin, and then used it for his own benefit, has not transgressed meilah.
No rabbinic extension was attached here, and the reason is worth noting. Blood in the Kidron lay where any passerby could help himself to it, whereas nesachim that had entered the shitin were beyond the reach of the public. Only kohanim could get at them, and only on the infrequent occasions when the shitin were cleared out. Since the Sages do not enact decrees to address circumstances that hardly ever occur, they left poured nesachim outside the scope of meilah. With nesachim, then, the leniency comes at the close rather than at the opening.