Rashi on Leviticus 11:38/רש"י על ויקרא י״א:ל״ח

1‎זרע‎‎ ‎על‎‎ מים‎ וכי יתן BUT IF ANY WATER BE PUT ON THE SEED, after it has been plucked, … it shall be unclean; for if you assert that there can be fitness to receive uncleanness whilst it is still attached to the soil, you will never have any seed which has not become thus fitted (Chullin 118b; cf. Sifra, Shemini, Section 8 2 and Rashi on v. 34).

2מים על זרע WATER ON SEED — whether it be water or any other liquids, whether these are on the seed or the seed falls into them — all this is derived in Torath Cohanim (from the wording) (Sifra, Shemini, Chapter 11 7-10).

3ונפל מנבלתם עליו AND ANY PART O. THEIR CARRION FALL THEREON, even after it has become dry from the water, because the Torah is particular only that the name “food” should be applicable to it. Consequently as soon as the fitness to receive uncleanness has once fallen upon it, it can never again be removed from it even if it becomes dry again (cf. Bava Metzia 22a).