Thy rod - Apparently the rod before described Exodus 4:2, which Moses on this occasion gives to Aaron as his representative.

A serpent - A word different from that in Exodus 4:3. Here a more general term, תנין tannı̂yn, is employed, which in other passages includes all sea or river monsters, and is more specially applied to the crocodile as a symbol of Egypt. It occurs in the Egyptian ritual, nearly in the same form, "Tanem," as a synonym of the monster serpent which represents the principle of antagonism to light and life.

Show a miracle for you - A miracle, מופת mopheth, signifies an effect produced in nature which is opposed to its laws, or such as its powers are inadequate to produce. As Moses and Aaron professed to have a Divine mission, and to come to Pharaoh on the most extraordinary occasion, making a most singular and unprecedented demand, it was natural to suppose, if Pharaoh should even give them an audience, that he would require them to give him some proof by an extraordinary sign that their pretensions to such a Divine mission were well founded and incontestable. For it appears to have ever been the sense of mankind, that he who has a Divine mission to effect some extraordinary purpose can give a supernatural proof that he has got this extraordinary commission.

Take thy rod - This rod, whether a common staff, an ensign of office, or a shepherd's crook, was now consecrated for the purpose of working miracles; and is indifferently called the rod of God, the rod of Moses, and the rod of Aaron. God gave it the miraculous power, and Moses and Aaron used it indifferently.

When Pharaoh shall speak unto you, saying, show a miracle for you,.... To prove that they came from God, the Jehovah they said they did, and that they were his ambassadors, and came in his name, and made the demand for him; which when he seriously reflected on things, he would be ready to require, hoping they would not be able to show any, and then he should have somewhat against them, and treat them as impostors:

then thou shalt say unto Aaron, take thy rod; the same that Moses had in his hand at Horeb, and brought with him to Egypt; this he had delivered into the hand of Aaron, who was to be his agent, and with this rod do signs and wonders as he did, and on account of them it is sometimes called the rod of God:

and cast it before Pharaoh, and it shall become a serpent; as it became one before at Horeb, when Moses by the order of God cast it on the ground, and afterwards became a rod again, as it now was, Exodus 4:2 Hence Mercury, the messenger of the gods with the Heathens, is represented as having a "caduceus", a rod or wand twisted about with snakes (p).

(p) Vid. Chartar. de Imag. Deorum, p. 136. imag. 48.

When Pharaoh shall speak unto you, saying, Show a miracle for you: then thou shalt say unto Aaron, Take thy rod, and cast it before Pharaoh, and it shall become a serpent.

7:9 Say unto Aaron, Take thy rod - This Moses ordinarily held in his hand, and delivered it to Aaron upon occasion, for the execution of his commands.

9. When Pharaoh shall speak unto you, &c.-The king would naturally demand some evidence of their having been sent from God; and as he would expect the ministers of his own gods to do the same works, the contest, in the nature of the case, would be one of miracles. Notice has already been taken of the rod of Moses (Ex 4:2), but rods were carried also by all nobles and official persons in the court of Pharaoh. It was an Egyptian custom, and the rods were symbols of authority or rank. Hence God commanded His servants to use a rod.

7:8-13 What men dislike, because it opposes their pride and lusts, they will not be convinced of; but it is easy to cause them to believe things they wish to be true. God always sends with his word full proofs of its Divine authority; but when men are bent to disobey, and willing to object, he often permits a snare to be laid wherein they are entangled. The magicians were cheats, trying to copy the real miracles of Moses by secret sleights or jugglings, which to a small extent they succeeded in doing, so as to deceive the bystanders, but they were at length obliged to confess they could not any longer imitate the effects of Divine power. None assist more in the destruction of sinners, than such as resist the truth by amusing men with a counterfeit resemblance of it. Satan is most to be dreaded when transformed into an angel of light.