Or HaChaim on Exodus 4:3/אור החיים על שמות ד׳:ג׳

1ויהי לנחש, it had turned into a snake. G'd hinted to Moses that the forces of the קליפה are characterised by the symbol snake. We know from the original serpent that it represents סם, i.e. something potentially poisonous. G'd wanted to teach Moses that his hands possessed the power to neutralise the power of that serpent and to turn it into a harmless piece of wood. If, however, Moses would not keep firm control and allow it to escape from his hands, he would unleash all the latent powers of evil which are controlled by the serpent. This would become so threatening that Moses would flee from it [although he stood on consecrated ground, Ed.]. When G'd asked Moses: "what is this in your hand?" He wanted Moses to understand that the staff was biologically analogous to a certain kind of desert-mouse. If Moses were to even loosen his grip on his staff it was liable to develop into a serpent just as earth can turn into a certain species of mouse. [In Talmudic times there was a widespread belief that this specimen of mouse was half flesh and half earth, and developed out of the earth. This belief has halachic implications regarding the laws of ritual impurity, compare Chulin 9,6, Ed.] Moses replied to G'd's question that he was certain that what he held in his hand was a staff, i.e. one hundred per cent wood, without potential to develop into a living organism. When G'd made him throw the staff down to earth, He demonstrated that not only did the staff contain the potential to develop into a living creature, but it did not do so only very gradually, such as the desert-mouse, but it did so in a single moment.

2The reason G'd had said: "throw it to the ground," was that in the eyes of the serpent the earth assumes great significance since it derives all its needs for survival from the earth, and earth (dust) is its exclusive habitat (Genesis 3,14). G'd taught Moses many lessons by means of this small demonstration. Amongst other things Moses was to demonstrate this very miracle before the eyes of the Israelites.

3When G'd instructed Moses: "stretch out your hand," He wanted Moses to realise that though he thought that he had already lost control over what had been his staff, he still possessed the power to master what had become a serpent and to turn it into a harmless piece of wood again.

4G'd instructed Moses ואחוז בזנבו, instead of ואחוז בו. This was to teach him an additional lesson. Normally, people are afraid of the poisonous head of the snake and when they try to grab it they try to get hold of its head and crush it in order to neutralise its poisonous bite. G'd told Moses that it would suffice to grasp the tail of the snake and he would not have anything to fear. When the Torah nevertheless describes Moses as ויחזק בו "he took hold of it," and not as ויחזק בזנבו, "he took hold of its tail," this does not mean that Moses did not obey G'd's instructions but that he took hold of the nearest part of the snake without fear. When the Torah goes on to describe that ויהי למטה בכפו, "it turned into a staff in his palm," this means that it ceased being a living organism as soon as Moses' palm touched it i.e. ויהי, "it remained as a staff only."