Maseches Challah, Chapter 4, Mishnah 3. This mishnah deals with two batches of dough, neither of which contains the amount that obligates in challah, and their owner wishes to join them together in order to reach the required amount - except that they do not touch one another, and something else separates between them. The question before us: when do two batches of dough combine even though there is a separation between them, and when do they not combine?
The first case - rice or Terumah in the middle:
Before us are two kavim, neither of which on its own reaches the amount obligated in challah, but together they contain the required amount. Both are of one species - wheat flour. In the middle, between the two batches of dough, something else is lying:
A kav of rice - rice is not obligated in challah.
A kav of Terumah - Terumah too is not obligated in challah.
About this the mishnah says: "ein mitztarfin" - they do not combine. The two outer batches of dough, even though both are of wheat, do not combine with one another, because what separates between them is something that is not obligated in challah at all.
The second case - something whose challah was already taken in the middle:
Here we are speaking of a separating substance that does have a connection to the obligation of challah: a batch of wheat flour dough that was obligated in challah, except that challah has already been separated from it. In this case the mishnah says: "mitztarfin, shekvar nischayev bachallah" - they combine, since it was already obligated in challah. Since the dough lying in the middle is counted among the things that have a connection to challah, and challah was in fact separated from it, it has the power to join the two batches of dough on either side of it to one another.
In summary: our mishnah establishes that when something separates between two batches of dough, neither of which contains the required amount, everything depends on the separating substance itself: if it is something that is not obligated in challah - such as rice or Terumah - the batches of dough do not combine; and if it is something that became obligated in challah and its challah has already been taken - it does join them together.