The people spoke out against אלהים [Elohim] and Moshe [Moses], 'Why did you take us out of Egypt to die in the desert? There is no bread and no מים [mayim, water]! Our נפש [nephesh, soul] is getting disgusted with this insubstantial food.' [They tired of the manna from God, calling it "tasteless." In other words, they craved sensation, and began to break the Law.]
יהוה [Jehovah] sent שרף [seraph, fiery] נחש [nachash, serpents] against the people, and when they [tempting serpents of Eden] began biting the people, a number of Israelites died.
The people came to Moses and said, 'We have sinned by speaking against יהוה [Jehovah] and you. Pray to יהוה [Jehovah], and have Him take the snakes away from us.'
God [Iod-Havah Elohai (Binah)] said to Moses, 'Make yourself [the image of] a snake, and place it on a pole. Everyone who is bitten shall look at it and live.'
Moses made a brazen [Hebrew nechosheth: brass] snake [Hebrew nachash] and placed it on a high pole. Whenever a snake bit a man, he would gaze at the brazen snake and live. - Quoted from Numbers 21
In synthesis, there are two forms of "serpents:"
Brass is made by combing two metals: copper (feminine) and tin (masculine). This story hides the secret of Alchemy: how to purify base metals and make them pure by raising a serpent (kundalini, or transmuted sexual energy) upon a staff (the spine).