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 4:2-4 ) Hence Mercury, the messenger of the gods with the Heathens, is represented as having a "caduceus", a rod or wand twisted about with snakes F16.