Eruvin Chapter 7, Mishnah 5. This mishnah discusses using a pile of straw as a partition between two courtyards.
The words of the Mishnah and their explanation:
"Matban shebein shtei chatzeiros" - a matban is a pile of straw (from the word teven, straw), and we are speaking of a pile standing between two courtyards.
"Gavoah asarah tefachim" - this is the height required for a valid partition.
"Me'arvin shnayim v'ein me'arvin echad" - the two courtyards make two separate eruvei chatzeiros, and they cannot join together in a single eruv, since the pile of straw serves as a partition between them.
A partition made of straw as opposed to nullifying a partition:
In Mishnah 3 we learned that straw placed in a ditch does not nullify the ditch's status as a partition, because we are concerned that it will be taken away. It does not have the power to nullify a partition, and it is not permanent enough to prevent the ditch from serving as a partition. But when it comes to creating a partition, being a wall in its own right, as long as the wall of straw stands in place it is a wall.
"V'eilu ma'achilin mikan v'eilu ma'achilin mikan":
The Mishnah teaches that it is permitted to feed animals from this straw on Shabbos, and the people of each courtyard feed from their own side. The Gemara explains that we are not concerned that the wall of straw will be diminished and fall below ten tefachim without anyone noticing, while the people of the courtyards continue to carry. There are two reasons for this:
Animals do not eat such a large quantity on Shabbos.
For a problem to arise, the partition would have to be reduced below ten tefachim along a stretch of more than ten amos. In less than ten amos, the worst that can happen is that it will have the status of a doorway, and a doorway between two courtyards does not create a prohibition between them. Only a full breach of more than ten amos constitutes a problem, and it is not usual for animals to eat that much on Shabbos.
Nevertheless, the Gemara says that the people of the courtyard are not permitted to take from the straw and feed their animals themselves directly, because they are liable to take a significant quantity. But when the animal eats naturally on its own, it is not likely to eat that much.
"Nisma'et hateven me'asarah tefachim":
What is the ruling regarding a pile of straw that became lower than ten tefachim? This was said about a case where the diminishing occurred before Shabbos. In such a case - "me'arvin echad v'ein me'arvin shnayim" - and they can only make a single eruv. This necessarily refers to a case where the width is ten amos, and therefore there is no partition here, for in less than ten amos it would have received the status of a doorway.
In summary: In this mishnah we learned that a pile of straw ten tefachim high standing between two courtyards constitutes a partition, and therefore they make two eruvin and not one. Although straw lacks the permanence needed to nullify an existing partition, it is sufficient to be a wall in its own right as long as it stands. It is permitted to feed animals from the straw on both sides, since there is no concern that it will be reduced below ten tefachim along a stretch of more than ten amos, but one may not take from it by hand to feed them. And if the straw was reduced below ten tefachim before Shabbos across a width of ten amos, the partition is nullified, and they make one eruv and not two.