How did sonic turn into super sonic

Sonic turns into Super Sonic whenever he gets all seven of the Chaos Emeralds.

You can turn into Super Sonic in a regular level if you have at least 50 rings in Sonic the Hedgehog 2, Sonic the Hedgehog 3, Sonic & Knuckles, and Sonic 3 & Knuckles.

In later games, you can't turn super in normal levels, you can only be super during the final boss fight.