Demai, Chapter Seven, Mishnah Six
As we have learned on other occasions throughout this tractate, one cannot separate tithes from produce that is exempt from being tithed on behalf of produce that is obligated in tithes. Our mishnah discusses one practical implication of this principle.
1) There were before him two baskets full of tevel, and he said: Let the tithes of this [basket] be in that [basket], the first [basket] is tithed.
2) [If he said:] Let the tithes of this [basket] be in that [basket], and the tithes of that [basket] in this [basket], the first basket is tithed [whereas the second is not].
3) [If he said:] Let the tithes be so that the tithes of each basket be in the other, he has designated [the tithes of either basket].
Section one: The person has in front of him two baskets of untithed produce and for some reason, wants to separate tithes for one basket from another. He can state that the tithes of one basket will be taken out of the other. This is fine because both are untithed.
Section two: In this section, he wants to criss-cross the tithes, taking tithes for basket A from basket B and vice versa. In this section, the way he states this is not an effective of tithing the second basket because once he says the tithes of basket A are in basket B basket A is exempt from tithes. He cant subsequently use the produce in basket A as tithes for the produce in basket B, because one cant separate tithes from exempt produce (basket A) for obligated produce (basket B). Before he eats the produce from basket B he will have to separate tithes from it.
Section three: In this section, he makes one integral statement, as opposed to making a separate statement about one and then about the other. Since he simultaneously declares the tithes of both baskets, he is not separating tithes from exempt produce for obligated produce and the declaration works. Both baskets have been successfully tithed and he may eat from either of them.