The Serpent and the File (Greek Folk Tale)

A Serpent in the course of its wanderings came into an armourer”s shop. As he glided over the floor he felt his skin pricked by a file lying there. In a rage he turned round upon it and tried to dart his fangs into it; but he could do no harm to heavy iron and had soon to give over his wrath.

It is useless attacking the insensible.