Moses and Aaron show the sign of a staff turning into a snake and the Nile turning to blood. Pharaoh's heart is hardened because his magicians can perform tricks similar to these but far inferior to them. Because of the scale of the Nile turning to blood, God is demonstrating His full control over the world and over life (since life is in blood, Genesis 9:4). Although this presents the Law that God has mastery over all creation and is subject to His authority, God means to use this Law to turn people to His promises. The plagues punish the sin of the Egyptians so they will free the Israelites and allow them to go into the promised land. God uses the Law to redirect us to the Gospel.