It became a serpent This was proper, not only to affect Pharaoh with wonder, but to strike a terror upon him. This first miracle, though it was not a plague, yet amounted to the threatening of a plague; if it made not Pharaoh feel, it made him fear; and this is God's method of dealing with sinners; he comes upon them gradually.
Related Commentaries of Exodus 7:10
Exodus 7:10
10 And Moses and Aaron went in unto Pharaoh, and they did so as the LORD had commanded: and Aaron cast down his rod before Pharaoh, and before his servants, and it became a serpent.