Parah, Chapter 11, Mishnah 8. This section of the chapter deals with the eizov, the hyssop used for dipping into the mei chatas and sprinkling upon one who is tamei. Our Mishnah asks two practical questions about that hyssop: may it be used a second time for a different purification, and what happens when hyssop that was gathered for some other purpose becomes wet?
Eizov that has already served
"Eizov shehizah bo, kasher letaheir bo es hametzora." Hyssop with which one already sprinkled the mei chatas has not been used up. It remains fit to be used again, this time for the purification of a metzora, whose taharah also requires eizov. Having served once for one taharah does not disqualify it from serving for another.
When liquid falls on the eizov
The remainder of the Mishnah turns on the laws of susceptibility to tumah. Produce becomes capable of receiving tumah only once it has been wet by one of the liquids that render it mekabel tumah, and only when it is designated as food. Wood gathered to be burned is not food, so moisture does not make it susceptible.
Suppose the hyssop was picked with firewood in mind, "lekato le'eitzim", and afterward some liquid spilled onto it, "venaflo alav mashkin". The Mishnah rules "menagbo vehu kasher": he dries the sprig off and may still dip it in the mei chatas. Since his intention at the time of picking marked it as wood rather than as something to eat, the wetting never gave it the capacity to contract tumah at all.
"Lekato le'ochlin, venaflo alav mashkin, af al pi shenigbo, pasul." If he gathered it as food and liquid fell on it, then even after it has dried out it is unfit. Once the hyssop was designated for eating, the liquid rendered it susceptible to tumah, and drying it afterward does not undo that status.
Gathering it for the chatas itself
"Lekato lechatas, kenilkato le'ochlin, divrei Rebbi Meir." If he gathered the hyssop specifically for use with the mei chatas, Rebbi Meir holds that its law is like hyssop gathered for food: it too becomes susceptible to tumah when liquid falls upon it.
The lesson of the Mishnah is that the fitness of the eizov depends not only on what it is, but on what the person had in mind when he picked it. The same sprig, gathered with one intention, stays fit after a spill; gathered with another, it is lost to the sprinkling even after it dries.