Bava Kamma, Chapter 2, Mishnah 3. The Mishnah continues explaining the five ways in which animals are considered forewarned. In the first and second mishnayos, we learned about the two primary categories of damage, Regel and Shen, and now the Mishnah comes to distinguish between them.
"Hakelev vehagedi shekafztu merosh hagag veshavru es hakelim" - The dog and the goat that jumped from the top of the roof and broke the vessels:
If a person kept a dog or a goat on the roof of his house, and they jumped from the roof and landed in another's courtyard and broke vessels there - "meshalem nezek shalem, mipnei shehen mu'adin" - he pays full damages, because they are considered forewarned. It is normal for goats and dogs to jump, this is their standard behavior, and this is how they get around and go about their business, including jumping from roofs. Therefore, this is a subcategory of Regel, and since it is an expected damage, if it happened in the private property of the damaged party, the damager, the owner of the dog or goat, is liable for full damages.
"Hakelev shenatal chararah" - The dog that took a baked cake:
A chararah is a flat baked cake, whose dough is placed directly on the coals of a fire. If a dog took such a cake, it stands to reason that the coals themselves stuck to it, and if they are still glowing and hot, they are dangerous. "Vehalach legadish" - and went to a grain stack - the dog carries the baked cake to the grain stack to eat it there. "Achal hachararah vehidlik es hagadish" - it ate the baked cake and ignited the grain stack - here, two damages occur:
Eating the baked cake: A classic case of Shen damage, as it is normal for dogs to eat this type of food.
Burning the grain stack: The dog shook off the glowing coals that were stuck to the baked cake, the coals landed on the grain stack, and it was completely burned down.
Hence the ruling of the Mishnah: "Al hachararah meshalem nezek shalem, ve'al hagadish meshalem chatzi nezek" - for the baked cake he pays full damages, and for the grain stack he pays half damages - for eating the baked cake, the dog's owner is liable for full damages, as this is Shen damage since it is the dog's normal behavior; whereas for the grain stack that was burned, he pays half the loss caused to the owner of the grain stack.
Why does he pay only half damages for the grain stack?
It is not because this is a case of Keren. For it to be considered Keren, the act would have had to be unusual, done out of mischief or malice, and that is not the case here: the dog is walking in its normal manner, and this is an example of Regel. Rather, it is not damaging directly with its body, but by its force - through something it moved and caused to happen.
Therefore, this falls into the category parallel to Tzeroros (pebbles), a law given to Moshe at Sinai that we possess: an animal that caused damage in the manner of Regel, during its normal activity, but not with its actual body and not with something attached to its body, but rather in an indirect way caused by its actions - such as kicking a stone, and here, scattering glowing coals and causing a fire - pays half damages. This is considered its force, with the same status as Tzeroros, and is a subcategory of Regel.
An additional explanation in the Gemara:
The Gemara brings another explanation: the dog did not shake off the coals that were stuck to the baked cake, but rather set down the cake, and the coals ignited the grain stack in the place where they were set down. According to this explanation, the spot where the dog ate the baked cake and which was burned directly from the baked cake and the hot coals - the law is full damages, since this is part of the dog's normal manner of operation and is not indirect damage; whereas for the rest of the grain stack, besides the part directly beneath the coals, he pays half damages.
The principle, therefore, remains the same: half damages are paid when the dog causes damage through its force, without the direct physical contact of its action.
In summary: In this Mishnah, we learned the distinction between damage caused directly by an animal's body and damage caused by its force. A dog or a goat that jumped from the top of a roof and broke utensils - full damages are paid, since jumping is their normal manner and they are considered forewarned for it, making it a subcategory of regel. A dog that took a baked cake and went to a stack of grain - for the cake that it ate, full damages are paid under the category of shen, and for the stack of grain that was burned, half damages are paid, since the damage was done indirectly by its force. This follows the law of tzroros (pebbles), which is a subcategory of regel.