Printed from http://tektonics.org/moswater.php
In the story of the Ten Plagues, Moses and Aaron turn all the water in the Nile into blood. The Egyptian magicians do the same thing, it says -- but the critics say, since Moses turns the water into blood, there's no water left for the Egyptians to do the same thing. So what's the answer?
The most popular way to answer this one is to point to places where the Egyptians could have gotten more fresh water for the magicians to turn into blood. Ex. 7:24 tells us that the people dug by the side of the river for fresh water. That's one possibility.
Even better, though: Fresh water from the Nile's source outside of Egypt would have kept flowing and replaced the blooded water within a matter of a couple of days. Not that it helps - who needed more blood?
It's worthwhile to note that it isn't assumable that the water assumed every characteristic of blood, including viscosity; as long as it looked, and smelled, like blood, it was enough for it to be called that descriptively: It is not as though samples were taken to detect corpuscles, and the Hebrew word had a precise scientific meaning for such things.
However, the answer I opt for is that when we are told that the Egyptians magicians "did the same" or "did so" by their arts, that this does not mean that they repeated Moses' performance, but rather than turning water into blood it was part of their known repetoire of tricks. The magicians would have had enough honor status in Pharaoh's eyes so that all they had to do was flip open a spell book and say, "Here, we have a trick like that already."
Since the Plagues were a contest of honor between the gods of Egypt and the God of the Jews, and the Egyptian gods already had the upper hand in the mind of the Pharaoh, there was no need for the magicians to turn any water into blood for the sake of proving that they could do it. An ancient record that said it could or was done would be sufficient when combined with their personal honor and authority. And that means that there's no need to worry about where they got the water they needed to do the trick.