ויאמר ד' אל משה אמר אל אהרן נטה את מטך והך את עפר הארץ והיה לכנם
Rashi explains that, like the plague of blood and frogs, Moshe could not catalyze this plague. Just as the water had protected him as an infant, the dust of the land protected him when he used it to conceal the Egyptian that he had killed. To smite the earth would have been an act of ingratitude on his part. This begs elucidation. The water saved Moshe – end of story. His basket was placed in the water and remained there until Bisyah, Pharaoh’s daughter, discovered it. The dust of the earth’s protection was short-lived, since it did not successfully hide…
