Under the leadership of Tarakasura, the asuras used to torment the devas. Tarakasura had received a boon from Brahma that he could only be killed by an offspring of Shiva. Indra and Brahma requested Shiva to produce a son, and he agreed. But though a hundred years passed, Shiva and Parvati remained immersed in each other. The world was nearing collapse and the devas, as per Brahma’s instruction asked Agni to distract the couple. Agni reluctantly approached the couple and increased the heat to distract them. Shiva gave his seed to Agni, who gave it to Ganga. Ganga left it in Meru as per Shiva’s advice and there the Shaivabhutas worshiped it. A thousand years later, a six faced child was born and that was Subramanya.

From Shiva’s seed was also created gold and silver, and then iron, copper and bronze, and from the dirt in the Ganga, tin, as per Ramayana Balakanda Sarga 37.