In everything that G-d taught Moses, He would tell him both the manner of contamination and the manner of purification. When G-d came to the laws concerning one who comes in contact with a dead body, Moses said to Him: "Master of the universe! If one is thus contaminated, how may he be purified?" G-d did not answer him. At that moment, the face of Moses turned pale.
When G-d came to the section of the Red Heifer, He said to Moses: "This is its manner of purification." Said Moses to G-d: "Master of the universe! This is a purification?" Said G-d: "Moses, it is a chok, a decree that I have decreed, and no creature can fully comprehend My decrees."
— Midrash Rabbah
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