The intent of the first sign.
Primarily, no doubt, the object was to empower Moses to show forth a sign easily, readily, without preparation, and so at any moment. He had come to the time of life at which he naturally carried a staff. That he should be able at his will to transform that dead piece of vegetable matter into an active, living organism, would show him endued with supernatural power over both the vegetable and animal worlds, and give him a means, always ready to his hand, of demonstrating the truth of his mission. This alone was a great matter. But the fact that his rod became a serpent, rather than any other living thing, was specially calculated to impress the Egyptians. In one form, the serpent with them meant "a king," or "a crown;" and the change of a staff into a snake would typify the conversion of a shepherd into a monarch. In another form it was a sign for a "multitude," and the transformation might remind them that the single stock or stem of Jacob was now become "millions." The great serpent, Apap, moreover, held a high position in their mythology, as powerful to destroy and punish, whence they might the more fear one who seemed able to create serpents at his pleasure. The Israelites would perhaps view the staff as a rod to smite with, and connect its change into a serpent with the notion that when reds or whips were not thought severe enough, rulers chastised with "scorpions" (1 Kings 12:11). Altogether, the sign, if viewed as a type, was threatening and alarming; perhaps the more so on account of its vagueness. Forms ill-defined, seen through mist, affright men more than those which are clear and definite.