Exodus 7:8-12 Then the Lord said to Moses and Aaron, “When Pharaoh says to you, ‘Prove yourselves by working a miracle,’ then you shall say to Aaron, ‘Take your staff and cast it down before Pharaoh, that it may become a serpent.’” So Moses and Aaron went to Pharaoh and did just as the Lord commanded. Aaron cast down his staff before Pharaoh and his servants, and it became a serpent. Then Pharaoh summoned the wise men and the sorcerers, and they, the magicians of Egypt, also did the same by their secret arts. For each man cast down his staff, and they became serpents. But Aaron's staff swallowed up their staffs.

These verses specifically refer to the rod turning into a serpent as a miracle, but somehow the Pharaoh's sorcerers were able to recreate this miracle with their "secret arts". What were their secret arts? How did these magicians have powers comparable to God? If it was God's intent to convince the Pharaoh of Moses' divine authority, why wouldn't he begin with an example that could not be replicated (or seemingly replicated) by any man, angel, or demon?