We're learning Mishnayot Ma'asrot, and we're in Perek Dalet (Chapter 4). Today we'll start from Mishnah Gimel (Mishnah 3). So again, the discussion here is that we know that until the product is chayav (obligated) in Ma'asrot, you're allowed to snack on it. The question then becomes: What's considered snacking? The Mishnah says you're taking olives from the vat.
Now, what is the purpose of the Ma'atan? The Bartenura says they collect the olives, and they become soft. Before you can extract the oil from them, you need to soften them up. So the vessel that you soften them up in is called the Ma'atan. If somebody takes olives from the Ma'atan, he can take one at a time, dip it in the salt, and eat it. And eat it. And eat it. Clearly, that would be considered snacking. However, if he salts an olive and places it in front of him, then salts the second olive and places it in front of him, then he's not snacking anymore. That would be considered Achilat Keva (fixed eating). And of course, at that point, he would be obligated to take Ma'asrot before eating.
Rabbi Eliezer has a different way of viewing this. It's very interesting. Remember, we learned in a later Mishnah, actually, that when a person has the ability to take the leftovers and throw them back into the box, that's considered snacking. It's considered temporary. Why? Because the logic is that whatever's still in the box, you're allowed to snack on. When you take it out of the box and you're able to put it back in, what you took out has the status of the box itself. So just as you could snack on things in the box, the stuff that you took out isn't going to have a different status.
Therefore, the Mishnah is going to say now: Let's say the Ma'atan, this barrel that you have the olives in, has olives that are Tahor (ritually pure), but the guy is Tameh (ritually impure). Once he takes out two olives, he has made these olives Tameh. He can't throw them back into the pile, because he's going to make the whole thing Tameh. So then already his removal separates them from the box. You should be able to snack from the box, but since these are separate from the box, you cannot snack on them anymore, and therefore you have to take Ma'aser (tithe).
Now, if you took from a Tameh Ma'atan, what's the difference? He's able to throw them back in because he's Tameh; the Ma'atan is Tameh. Therefore, these items that you removed are not separate from the Ma'atan itself. And since I could say, “I could snack from the Ma'atan,” I could snack from the ones that I took out. The Mishnah explains that since you're able to return the leftovers, if you could return them, they have the status of the contents of the Ma'atan.
Let's read the Bartenura on that. The guy who's taking the olives is Tameh. He cannot return the leftovers, because otherwise he'll make everything else Tameh. Whatever he touched became Tameh. So he has separated what he has in his hands from the Ma'atan. Therefore, the items that he took in his hands are obligated in Ma'aser. He can return the leftovers, and since he can return them, he hasn't separated what he took in his hands from the Ma'atan itself. And since he can snack from the Ma'atan, he's able to snack from what he took as well.