Then Moses made known to him that God would turn the water into blood, if he refused to let Israel go. In the warning we can discern the difference between God and man. When a mortal harbors the intention to do an injury to an enemy, he lies in wait for the moment when he can strike an unexpected blow. But God is outspoken. He warned Pharaoh and the Egyptians in public whenever a plague was about to descend, and each warning was repeated by Moses for a period of three weeks, although the plague itself endured but a single week.