Commentary for Bava Kamma 2:3

A dog or a goat jumped from a roof etc... A thin cake and sometimes that coals attach to it. And When they are eaten in the field and they ignite the field with that coal, behold in his eating of the cake he is acting normal and he pays full damages. And for lighting the field, it's different, and therefore he pays half damages. (other versions translate differently- saying he pays half because it is pebbles).

מפני שהן מועדין – to jump, and we are speaking of in the domain of the one who suffered damages, for it is a derivative of the “foot.”

If a dog or a goat jumped from a roof and broke vessels, [the owner] must pay full damages, since they are attested dangers.
A dog that took a cake [while there was a cinder attached] and went to a stack of grain and ate the cake and burned the stack of grain, For the cake [the owner] pays full damages And for the stack of grain [the owner] pays half damages.
This mishnah deals with damages done by either dogs or goats. The important principle to remember when reading this mishnah is that a person is obligated full damages when the damager is an attested danger (muad) and only half damages when the damager is an unattested danger (tam). One is obligated to guard animals in one’s possession which attested dangers and therefore if one fails to do so, the restitution which must be made is higher. Furthermore, we have learned several times already, that animals can be muad for those actions which they are likely to do and tam for those that they are unlikely to do. For instance in the previous mishnah we learned that an ox is muad to eat grain but tam to eat clothing. Over the first the ox’s owner will pay full damages and over the second half damages.
Evidently dogs and goats were likely to damage by jumping off roofs. Therefore, there owner must prevent them from doing so and if she fails, she is obligated to pay full damages.
In the second section of the mishnah we learn that dogs are likely to eat cake (I know my dog is!), but unlikely to burn down stacks of standing grain. Therefore for the cake one is obligated for full damages and for the stack only half damages.
Questions For Further Thought
 Is there a new principle which we are learning from the second clause of the mishnah? In other words, what might you have thought had not this clause been taught?

על החררה משלם נזק שלם – for this is the “tooth” in the domain of the one who suffered damages.

ועל הגדיש משלם חצי נזק – they are like pebbles, and the decided Halakha is that he should pay one-half damages.