Library Theodoret Questions on Exodus
Questions on Exodus 20: How the magicians also turned water to blood
If all the water was changed into blood, how did the enchanters of the Egyptians do likewise with their sorceries?
The sea also bordered them, and only the drinkable water was changed into blood. They were therefore able to convey seawater into the palace and change it to the color of blood—God clearly allowing them to work for the reasons we have stated. One must also note this: that he brought the frogs out of the waters, the gnats out of the dust, and the dog-fly from wherever he wished, so that they might learn how easy it is for God, even without earth and without waters, to bring forth what he wills. For this reason he also foretells the punishments, so that they may not guess these things happen of themselves, but know they are God-sent plagues.
Greek: Theodoret of Cyrus, Quaestiones in Octateuchum / Questions on Exodus (CPG 6200.02; cf. Fernández Marcos and Sáenz-Badillos; also PG 80, cols. 225–298). English: study translation for BWAB Library—not the copyrighted Robert C. Hill (CUA Press) translation. The full Questions on Exodus (72 questions) is hosted here.