parshat Va'era

The first plague was the plague of blood. Besides the obvious inconvenience, to say the least, of the plague, what significance was there in the choice of blood for this first calamity to befall the Egyptians?

Moses, who dared to request that God change His "Attribute of Judgement" to the "Attribute of Mercy," merited having God change the ways of nature in response to his prayers, and the decree of exile was thus nullified before the appointed time.