(Bamidbar 19:17) "And they shall take for the unclean one from the earth of the burning (of the heifer) for cleansing." R. Shimon said: Is it earth? Is it not ashes? Why does Scripture depart from its usual meaning? To formulate an identity (gezeirah shavah). It is written here "earth," and, elsewhere, "earth," (Bamidbar 5:17). "Just as there, "earth" on the water, so, here, "earth" on the water. And just as there, if the (taking of the) earth preceded the (taking of the) water, it is valid, so, here. Variantly: If it (the ashes) changed its appearance (to that of earth) it is (still) valid.