In the Book of Exodus, Moses's staff became a serpent when he threw it on the ground.
The sign appears when God commissions Moses at the burning bush. Moses questions whether the Israelites will believe that God has appeared to him, so he is given signs to demonstrate divine authority. When he throws down his staff, it becomes a serpent; when he grasps it by the tail, it becomes a staff again.
Exodus also describes a second sign involving Moses's hand becoming diseased and then restored, followed by a third sign involving water from the Nile becoming blood. These signs are intended to support Moses's message to the Israelites and Pharaoh.
The passage is sometimes remembered as a snake trick performed before Pharaoh, but the first transformation occurs during Moses's commissioning in the wilderness. Before Pharaoh, Aaron's staff is also associated with a serpent, creating a related but distinct scene.