The prophet Musa (علیہ السلام) in response to his demand, threw down his staff on the earth, instantly it turned into a serpent: فَإِذَا هِيَ ثُعْبَانٌ مُّبِينٌ. The word 'thu` ban' in Arabic signifies a huge serpent. The use of the word 'mubin' as an adjective which means 'clear or prominent' is quite descriptive of the fact that this miraculous event took place manifestly before the eyes of the people of the Pharaoh, and it was not performed secretly in a hidden or secluded place as is usually done by magicians. In some historical traditions it has been cited on the authority of the Companion Ibn ` Abbas ؓ that as the serpent moved towards Pharaoh, he jumped from his throne and sought shelter near the prophet Musa (علیہ السلام) and many of his courtiers died of extreme fear. (Tafsir Kabir)

Transformation of the staff into a real serpent is not, in fact, totally impossible as it apparently seems. It is, however, surprising due to being unusual. The miracle has to be an unusual act, beyond the power of a common individual. Allah shows the miracles through His prophets to make people understand that they possess some divine powers and are true prophets of Allah.