Before us is Mishnah 3 in chapter 4 of Maseches Maasros. The coming mishnayos all deal with a single question: a person takes a small quantity of produce from a batch whose work has not been completed, and performs a certain action on that small quantity - can that quantity acquire an independent status of "its work has been completed," as a sub-group within the whole, and become obligated in maaser before eating, even though this is nothing more than a casual snack?
"HaNotel zayis min hama'atan":
The Mishnah discusses a person who takes a few olives out of the ma'atan. The ma'atan is the vessel in which olives were placed after being picked, so that they would heat up and ferment in their own juice, soften and age somewhat, thereby making the extraction of oil far easier. As long as the olives are lying in the ma'atan awaiting pressing, their work has certainly not been completed. The question is what their status is when a small quantity is taken out of them.
To this the Mishnah says: "tovel echad echad bamelach v'ochel" - he may take them one by one, dip each one in salt and eat it, and this is permitted casual eating. It is worth being precise: we are speaking of dipping one olive at a time for immediate eating, and not of salting intended to dry the olives and pickle them, which is a completed act of work.
The Mishnah adds: "im malach v'nasan lefanav - chayav". When a person salts several olives one after another and sets them down before him to eat later, it turns out that he has created for himself a small portion that stands on its own. This portion is no longer designated for pressing into oil, but for eating, and with that it has reached the completion of its work.
The underlying principle resembles the laws of the meal that we learned earlier, which deal with combining and gathering several items together: from the moment more than one salted olive is lying before him, this is no longer eating from the hand to the mouth, and the obligation to tithe before eating takes effect.
The view of Rabbi Eliezer:
Rabbi Eliezer adds a further consideration, based on the question of whether the one taking the olives intends to return the remainder to the ma'atan. A person who is tamei and takes olives from a ma'atan whose contents are tahor will never return what is left in his hand to the ma'atan, since by doing so he would render the entire large quantity tamei. Since he does not intend to return them, the quantity he took is a separate portion from which there is no return, and it therefore became obligated in maaser immediately.
"min hama'atan hatahor - chayav" - one who takes from a tahor ma'atan while he himself is tamei, the quantity he removed must be tithed immediately, since it would never occur to him to return it.
"umin hatamei - patur, mipnei shehu machzir es hamosar" - one who takes from a ma'atan whose contents are tamei, the small quantity is exempt, since if he does not eat everything he took in his handful, he will readily return the remainder to the ma'atan.
The halachah, however, does not follow Rabbi Eliezer, but rather the anonymous Mishnah, the view of the Tanna Kamma. That is, we do not take the consideration of returning into account at all: as long as the person is not gathering several salted olives before him, taking a handful is considered casual eating, and he may eat from it without tithing.