Parah, Chapter 9, Mishnah 3. This chapter is devoted to the ways in which the sanctified water, the water mixed with the ashes of the parah adumah, becomes unfit for sprinkling. Our mishnah turns from things that fall into the water to living creatures that come and drink from it.
The Mishnah begins: "Shasas mehen behemah o chayah, pesulin" - if a domesticated animal or a wild animal drank from the water, it is unfit.
The reason such a creature ruins the water is that while it drinks it releases saliva back into the flask, and that returning moisture disqualifies the sanctified water. From here the Mishnah goes through the various categories of creature and asks, in each case, whether its manner of drinking causes it to give something back to the water.
Birds
Regarding winged creatures the Mishnah rules: "Kol ha'ofos poslin", every species of bird spoils the water, and then registers one exception, "chutz min hayonah", the dove is left out of the rule, "mipnei shehi motzetzes", since the dove takes in liquid by suction.
Birds generally drink in a way that leaves saliva behind in the water. The dove is the exception: it draws the water in by sucking, so nothing of its own flows back into the flask, and the water remains fit.
Creeping creatures
Turning now to the small crawling creatures, the Mishnah rules: "Kol hasheratzim einan poslin", as a class they leave the water fit, and again one exception is registered, "chutz min hachuldah", the weasel is different, "mipnei shehi melakekes", since it takes its water by lapping with the tongue.
Here the rule is reversed. As a class, these small creatures do not spoil the water when they drink from it. The weasel is singled out because of the way it laps at the water with its tongue, which brings its saliva back into what remains in the flask.
Two further opinions
Rabban Gamliel adds a further creature to the list. "Rabban Gamliel omer", he holds that "af hanachash", a snake as well ruins the sanctified water, "mipnei shehu meiki", because as it drinks it forces liquid back out of its mouth into what is left in the flask.
"Rabbi Eliezer omer: af ha'achbar" - Rabbi Eliezer says that a mouse as well renders the water unfit.
The halachah does not follow Rabbi Eliezer here. If a mouse drank from the sanctified water, the water remains fit for use.
The mishnah teaches us to look not merely at the fact that a creature drank, but at how it drinks. The same act of drinking, done by a dove and done by a weasel, has opposite consequences for the flask of mei chatas, and the halachah weighs each creature's manner precisely.