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1. The Beraisa lists many things that can stop Tum'ah from advancing.
2. The Beraisa also lists things that cannot stop Tum'ah from advancing.
3. The Gemara explains how animals and birds can be considered a permanent fixture to stop Tum'ah.
4. A person should not put the opening of his oven less than four Amos away from his ceiling when a neighbor lives on the upper floor.
5. The Mishnah explains under what conditions a person may protest against his neighbor's work in his property.
A BIT MORE
1. The Gemara earlier discussed items that -- when placed in a window -- prevent Tum'ah from a dead body from passing from one house to the other. The Beraisa lists many more items, such as grass, Nochrim, salt, earthenware, and a Sefer Torah.
2. The list includes items such as snow, hail, and water.
3. The Gemara says that while they are generally not considered to be a permanent fixture, when they are tied up they are considered permanent, and they therefore stop Tum'ah.
4. This is so that he not cause damage to his ceiling, which is also his neighbor's floor.
5. If there are too many people coming in and out of the neighbor's yard to a store located in the house, the other neighbors may protest. If there is noise from the sound of his work, they may not protest, as a person is allowed to work (to a certain degree) in his house.
Next Daf Index to Revach for Maseches Bava Basra