Tosefta sobre Números 19:9

There is a stringency with a neveilah (i.e., an animal carcass) that there is not with the waters of purification (Num. 19:9), and [there is a stringency] with the waters of purification that there is not with a neveilah. That a small quantity of the waters of purification is impure, and a small quantity of neveilah is pure. A stringency with neveilah is that the waters of purification purify that which is impure and contaminate that which is pure, but they do not join together impurities, which is not so with neveilah. [Moreover,] the waters of purification that dried out, behold, they are [permanently] pure, [but] a neveilah that dried out, if it is able to be soaked and thus able to return to its former status and join [with an object susceptible to impurity], behold, it is impure.

Said Rabbi Nechemiah, why is everyone trusted as to [the purity of the articles involved in] the purification rite [involving the red heifer] and as to consecrated food ("נאמנין על בהחטאת ועל הקדש", see Lieberman), but not everyone is trusted as to Terumah? So that people will not say, "Behold, I am building an altar for myself," or "Behold, I am burning a red heifer for myself," as it is said (Num. 18:7), "And you (=Aaron) and your sons with you shall safeguard your priesthood in every matter [regarding] the Altar..." (See Chag. 22a:15.) One might say that [the word "safeguard" includes] even [Terumah,] Terumat Ma'aser (the priestly portion taken from a Levite's tithes) and Challah (the portion of dough set aside for the Kohen), [and thus] the Torah comes to teach "... and to [that which is] within the Curtain." What is special about that which is "within the Curtain" is that it is not familiar to [the non-priestly class of] Israelites [who are forbidden from entering the area beyond the Curtain], which thus excludes Terumah and Terumat Ma'aser and Challah, with which Israelites are familiar. Rabbi Yehuda says, behold, it says [in regards to the red heifer] (Num. 19:9), "and it shall be for the congregation of the Children of Israel to safeguard it as the waters of purification...", such that everyone should be trusted as to safeguarding it. [If there is] a doubt as to the status of an am ha'aretz as regards to [the impurity of] treading or movement [of a zav (see Niddah 43a:3)], [they are deemed] pure as to matters of chullin and impure as to matters of Terumah.