Maasrot, Chapter Four, Mishnah Three
Our mishnah deals with olives that were put into a vat in order to soften and prepare them for the pressing process.
1) One who took olives from a vat may dip them one at a time in salt, and eat them.
2) But if he salted them, and put them in front of him, he is liable [for tithes].
3) Rabbi Eliezer said: from a pure vat he is liable but from an impure [vat] he is exempt because can put back the leftovers.
Section one: Salting the olives one at a time and eating them without tithing is okay because this individual salting does not make them liable for tithes.
Section two: However, if he salts several olives which he took from the vat, then he cant eat them without tithing. In other words, salting them together makes them liable for tithes.
Section three: According to Rabbi Eliezer if the vat was pure he cannot eat the olives without tithing them. The reason for this is that these olives have become susceptible to impurities because they came into contact with the olive oil inside the vat. If the purity of the olives in the vat has been preserved then he wont want to put the olives back because the olives that he touched may have become impure. Since he wont want to put them back, that which he takes out is be liable for tithes. If, in contrast, the vat was not pure, then he can put the olives back. The olives that he took out are not considered to be gathered together and they can be eaten without tithing.