Terumot, Chapter Two, Mishnah Six
This mishnah continues to discuss giving terumah from one species for another.
1) They may give terumah from olives [to be used] for oil for those to be preserved, but not from olives due to be preserved for olives [to be used] for oil.
2) [They may give] from unboiled wine for boiled wine, but not from boiled wine for unboiled wine.
3) This is the general rule: any two things which together are kilayim (mixed species) he should not give terumah from one for the other, even if one is superior and the other inferior.
4) But if they are not kilayim, then one may give terumah from the superior for that which is inferior, but not from the inferior for that which is superior.
5) If one does give terumah from the inferior for that which is superior, his terumah is terumah, except for zunin given for wheat, since these (zunin) are not food.
6) Cucumbers and melon count are one kind.
a) Rabbi Judah says: two kinds.
Section one: The olives that are used for oil are of a better quality than the olives that are going to be preserved and eaten. Therefore, one can take terumah from olives that will later be used for oil in order to exempt olives that are going to be preserved. However, one cannot do the reversetake terumah from olives that will be preserved from olives that will be used for oil.
Section two: The wine that is going to be boiled is the bad wine that will not taste as good as the unboiled wine (for a taste test, compare Golan to Manischewitz!). Therefore, he cant give terumah from boiled wine for unboiled wine, but he can give unboiled wine for boiled wine.
Section three: The mishnah now gives some general rules, which have been illustrated throughout the previous couple of mishnayot. First of all, anything which is kilayim with another species cannot be used as terumah for that species, even if one of the sets of produce is superior to the other.
Section four: If the species are not kilayim with one another, then one can give terumah from the superior species for the inferior species, as we learned before.
Section five: One should not give terumah from the inferior species for the superior species but if one does, the terumah is valid. For instance, if one gives terumah from boiled wine for unboiled wine, the terumah is valid. The only exception to this is zunin a type of species that grows between wheat and that is not kilayim with wheat (see Kilayim 1:1). Since zunin are inedible, they cannot be used for terumah.
Section six: In Kilayim 1:2 we learn that Rabbi Judah and the sages disagree whether melons and cucumbers are kilayim. Their dispute is brought here because according to the sages, one could give from one in order to exempt the other, whereas according to Rabbi Judah, one cannot.