And all the Egyptians dug round about the river for water to drink; for they could not drink of the water of the river.
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Augustine of Hippo
You asked … how, when all the water of Egypt was turned into blood, the magicians of Pharaoh found any [water] with which they could transform in like manner. This difficulty is usually solved in two ways. They did it either because some sea water could be brought or, what is more likely, because in that part of the country where the children of Israel were those plagues did not take place. In certain passages of that Scripture this is very clearly expressed, and it warns us what is to be understood even when it is not expressed.