Name: Sonic & Knuckles and Sonic the Hedgehog
Platform: Sega Genesis / Mega Drive
Region: USA
Media: Cartridge
Controller: Gamepad
Genre: Platformer
Gametype: Licensed
Release Year: 1994
Developer: Sonic Team
Publisher: SEGA
Players: 1
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Placing the first Sonic game, or most other Genesis/Mega Drive games, into the Sonic & Knuckles cartridge will display a screen                              depicting Sonic, Tails, Knuckles and Robotnik underneath scrolling text that repeats "No way? No way!". By pressing the A, B and C buttons together, a minigame based on the Chaos Emerald bonus levels is unlocked, called "Blue Sphere". There are over 100 million unique levels. The last level, "Special Stage", repeats itself once the level is beaten. Only by locking the original Sonic the Hedgehog or Sonic Compilation in the Sonic & Knuckles cart can one play through all the stages. The other Genesis/Mega Drive games will only play one level each, which will infinitely repeat. Despite being near randomly generated, the levels are playable for the most part, even if many bits and pieces of levels get recycled. On another note, some levels may be nearly impossible to complete while others are not as difficult.

http://en.wikipedia.org/wiki/Sonic_&_Knuckles