Why ten heads for Ravanasura?????

The interesting story behind the 10 heads of Ravan:

Considered the antagonist of the epic, Ravan, actually, was a great follower of Shiva, an inordinate scholar, an excellent ruler and a maestro of Veena. Attaining education, Ravan underwent a colossal penance to please Lord Shiva on the banks of river Narmada. Willing to please the Lord, Ravan axed his head. Each time that happened, the head grew back, which continued 10 times, pleasing Shiva. Shiva thus granted 10 heads to Ravan that he sacrificed. These 10 heads indicate the six shastras and the four vedas that Ravan mastered.

After winning over Lanka, Ravan again went to meet Shiva in Kailash, where he was denied entry by Nandi-Shiva's gatekeeper. Annoyed, Ravan teased him and hence a furious Nandi cursed him that his Lanka would be destroyed by a monkey! To prove his devotedness to Shiva, Ravan tried to lift Kailash. Angry Shiva placed a toe on the hill and Ravan's hand crushed beneath it, so painfully that his cry shook the world.

To please Shiva now, Ravan plucked out his intestines from stomach and played to please Lord Shiva, to which Shiva released him and gifted him a sword and gave him the name Ravan, which means "the one with terrifying roar".