Aaron threw down his rod before Pharaoh and it turned into a snake. Pharaoh summoned the wise men and sorcerers of Egypt... each cast his rod, and they turned into snakes; and Aaron's rod swallowed up their rods (Exodus 7:10-12)

Why did Aaron's rod swallowed the Egyptians' rods/serpents only after it had reverted back to its original form, rather than as a serpent itself?

This is to teach us that even when he wages war, the Jew is not a warrior. Even when he must destroy the serpents of the enemy, he is not a serpent himself, spewing poison and hate. His instrument of vengeance is as devoid of vengeful feeling as a lifeless stick.

(From the teachings of the Lubavitcher Rebbe)