(8) “And if they do not believe you or pay heed to the first sign, they will believe the second.
(9) And if they are not convinced by both these signs and still do not heed you, take some water from the Nile and pour it on the dry ground, and it — the water that you take from the Nile — will turn to blood on the dry ground.”
(8-9) And if they do not believe you or pay heed to the first sign, they will believe the second. And if they are not convinced by both these signs … the water will turn to blood on the dry ground: These signs have two purposes. Besides demonstrating to the People that Moses is sent to them from “Above,” they are also supposed to stimulate the Jewish People to leave Egypt. Each one of these three signs is more compelling than the previous one in the degree that they motivate the Jewish nation to yearn for the Exodus. Belief in signs (and, in this case, reliance on Moses’ leadership) depends to a large extent on psychological attitude and the level of one’s desire to see a promise fulfilled.
For those Jews who are focused on leaving Egypt, the first sign is sufficient, in that it demonstrates that it is indeed possible to “seize the Egyptian snake by the tail.”
For those who doubt whether it even pays to leave Egypt, the second sign demonstrates that without taking the proper precautions a person can contract leprosy.
Finally, for those who are determined to remain in Egypt, and therefore refuse to believe the first two signs of the Redemption, turning the water of the Nile to blood demonstrates that there is no more “water” in Egypt. Water is the source of life. Thus, remaining in Egypt is not just pointless but downright dangerous. This third sign, in the end, will convince even this last group.
But for Moses himself, the lesson here lies in the idea that blood represents the attribute of judgment. The meaning of this sign of water turning to blood is that Moses must be prepared to lead the Jewish people with hardheaded fierceness when necessary, as his position of commander-in-chief of the nation will sometimes demand (indeed, the story of the Exodus entailed not only the death of the Egyptian firstborns and of Pharaoh and his army, brought about by the Almighty; Moses too was later called upon to order the execution of the Jews who had worshipped the golden calf).