In this parsha, the Jews complain some more and get a Divine plague as retribution. After this plague, Moses is directed to make a brass serpent (Numbers 21:8-9). And after looking at the snake, the Jews are somehow cured. What's up with this? At first glance, it seems to be somewhere between superstitious and downright idolatrous.
Sforno tries to make sense of the matter, asserting that the brass snake is "from a material which implies 'burning' (i.e. such as brass) so that they would concentrate on the burning vapor (exhalation) of the serpent's mouth, this being akin to their iniquity and its resultant retribution, and thus they will repent."
In other words, the serpent is a kind of psychological tool; the Jews look at it, think of the snake and the vapor of the brass as somehow analogous to their sins, and then feel remorseful. I guess you might say that after looking at the snake's mouth they are reminded of the noxious vapors that came out of their mouths.
This passage is another matter of how our commentators take a crazy-seeming Torah passage and make sense of it. (Of course, sometimes they do the reverse, but that's another issue...)
