(Mishnah): There are four Avos Nezikin (primary categories of damagers): an ox, a pit, Mav'eh (Shen and Regel), and fire.

Harei (the stringency) of an ox is unlike that of Mav'eh, and Mav'eh is unlike Harei of an ox;

Harei of these two, which are alive, is unlike that of fire, which is not;

Harei of these three, which move and damage, is unlike that of a pit, which does not;

The common side of [all four] is that their nature is to damage, and you are obligated to guard them, and when they damaged, the damager must pay from Meitav (highest quality land).

(Gemara): 'Ox' refers to Keren (goring) - "v'Chi Yigof Shor Ish Es Shor Re'ehu...''

Question: This refers to a Tam. What is the source for a Mu'ad?

Answer: "Oh Noda Ki Shor Nagach Hu...''

A pit -[we learn from] "Ki Yiftach Ish Bor... Ba'al ha'Bor Yeshalem.''

Mav'eh - [we learn from] "Ki Yav'er Ish Sadeh Oh Cherem'';

"V'Shilach Es Be'iro'' is Regel, for it says "Meshalchei Regel ha'Shor veha'Chamor'';

And it says "Haser Meshukaso v'Hayah Leva'er'' - this is Shen. "Paratz Gedeiro v'Hayah l'Mirmas'' is Regel. (GILYON HA'SHAS cites RASHBA (2b), who asks what we expound from Ki Yav'er. He answers that it is a second source for Shen. One is liable not only if he sent it, rather, even if it went by itself.)

Fire - we learn from "Ki Setzei Esh u'Matzah Kotzim...''