And it shall come to pass, if they will not believe also these two signs, neither hearken unto your voice, that you shall take of the water of the river, and pour it upon the dry land: and the water which you take out of the river shall become blood upon the dry land.
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George Leo Haydock
Blood. This third sign had the same tendency as the former. It showed the cruel persecution inflicted upon the Hebrews, particularly in drowning their male infants; a cruelty which God would shortly revenge, by turning the waters of Egypt into blood, and by slaying the first-born and the army of the Egyptians. (Tirinus)