Moses stands at the water with the rod lifted, and God's words are simple and total: By this sign thou shalt know that I am the Lord (Exodus 7:17). Targum Pseudo-Jonathan on Exodus 7:17 preserves the thunder. The Nile, Egypt's lifeline, the river Pharaoh consulted each dawn like an oracle, is about to turn to blood.
The meturgeman leaves the promise naked and specific. No mysticism, no hedging — the waters of the river shall be changed into blood. The first plague is not a lecture on theology; it is a demonstration aimed at Egypt's most sacred and most practical asset. The Nile flooded the fields, watered the cattle, and filled the priests' vessels. Every one of those functions is about to stop.
Why blood? Because Pharaoh had ordered Hebrew boys drowned in this very river (Exodus 1:22). The water that swallowed Jewish children now gives back blood to the hand that poured them in. The sign is not arbitrary; it is a memory. The Nile becomes a witness.
The takeaway: God's signs are personal. The plague that shatters a tyrant is almost always shaped like the tyrant's own sin, handed back to him.