Parashas Masei · Passuk 35:18 · TT note 1
Chabad.org — English translation + Rashi
Torah Temimah points to Sifrei (TT note on “בִּכְלִי עֵץ”).
English: Iron, stone and wood are mentioned; the property of stone is not like that of wood nor wood like stone, nor both like iron. The common feature of all three is that [death comes] from lethal agents and a lethal act. Excluded is one who pushed him into fire or water or set a dog or snake upon him — his judgment is left to Heaven.
Binyan av / common feature — instruments that themselves strike versus indirect causation.
Capital liability under these verses applies to death by a lethal instrument in the killer’s hand; indirect methods (fire, water, animals) are left to Heaven’s judgment.