Variantly: "Ya'arof": R. Eliezer the son of R. Yossi Haglili says: "Ya'arof" connotes "killing," as it is written (Devarim 21:4) "And they shall break there the neck (ve'arfu) of the heifer in the river-bed." Just as the heifer (of the broken neck) atones for the spilling of blood, so, words of Torah.
"as se'irim on the herbage": Just as se'irim (he-goats) are brought for transgressions and atone for them, so, words of Torah.
Variantly: "Let my taking (i.e., Torah) break (ya'arof) as the rain": Moses (hereby) said to Israel: Do you know how much suffering (i.e., "breaking") I expended on Torah, how much I toiled in it, how much I wearied myself in it — viz. (Shemoth 34:28) "and he remained there (on Mount Sinai) forty days and forty nights." And I entered there among the angels, and among the heavenly creatures, and among the seraphs — one of whom can incinerate the entire world, viz. (Isaiah 6:2) "Seraphs were standing above Him" — I gave my life for it; I gave my blood for it. Just as I learned it in travail, so, you learn it (even) in travail. — But perhaps, just as you learned it in travail, so, shall you teach it in travail! It is, therefore, written "My word shall flow (tizal) as the dew" — See it (Torah) as "zol," ("cheap") a third or a fourth of a sela (i.e., "part with it easily" [to your disciples]).