The Sonic & Knuckles cartridge is designed to Lock-On with a Sonic 3 or a Sonic 2 cartridge. It will not work with non-Sonic cartridges.
Blue Sphere is a hidden minigame included on the Sonic & Knuckles cartridge, based on the Special Stages of Sonic 3 & Knuckles. It is playable when you place (almost) any Genesis cartridge other than Sonic the Hedgehog 2 or 3 into S&K's lock-on slot and press the A, B, and C buttons at the same time. The vast majority of Genesis cartridges you can do this with only give you a single stage.note If, however, you do this with the first Sonic game or Sonic Classics 3 in 1, you'll have access to a whopping 134,217,728 stages. note
The object is to collect the Blue Spheres scattered throughout the wrap-around stage. Red Spheres are to be avoided; touching one results in a Game Over. Star spheres are bumpers that will send you moving in reverse in the direction you came from until you move forward or make a turn. Yellow Spheres jump you five spaces forward. Rings can be found on the stage as well; collecting all Rings in a stage is required alongside getting all the Blue Spheres for a "PERFECT". Grids of Blue Spheres larger than 3 by 3 cells can be turned into Rings by collecting the Blue Spheres that make up the perimeter of the grid, and only those spheres.
Upon completion of the stage, you'll be given a twelve-digit password for use when you play the game with the original Sonic the Hedgehog or the aforementioned compilation re-release locked into the top slot of S3&K. It also appears as its own game option in Sonic Mega Collection, Sonic Mega Collection Plus (which gave the minigame an official name for the first time), and the Sonic and Knuckles Collection for the PC.
Blue Sphere makes a return in Sonic Mania as the Special Stage format for Star Posts. Since the "Catch the UFO!" stages are already used for Chaos Emeralds, Blue Sphere is instead used for collecting Medallions, which are required for unlocks. This version is designed to look like its 16-bit counterpart, though with a few liberties taken to make it look a bit more modernized such as smoother sprite animations and a 16:9 aspect ratio. Additionally, the standalone unlockable version of Blue Sphere adds Green Spheres and Pink Spheres: Green Spheres are double-durability spheres that each must be tagged to turn it into a Blue Sphere, and Pink Spheres that teleport you to a different location on the grid.
This game provides examples of:
- The Cameo: After completing a stage, the difficulty of that stage is displayed. If the difficulty rating is high enough, sprites of Mecha Sonic, EggRobo, and the animal buddies can appear.
- Cast Speciation: Subverted. You can play as Sonic or Knuckles, but there's no difference. All you can do is run and jump, and the speed you move at and your jump height is out of your control.
- Difficulty by Acceleration: You move faster the longer you take to complete the stage.
- Dungeon Bypass: Getting every ring in a stage in the full version of the game to get a "Perfect" skips you 10 stages ahead.
- Easter Egg: The game's entire existence. Technical difficulties with the palette prevented Sonic Team from doing Knuckles in Sonic 1, so they put this game in. The game's existence and the button combination required to play it weren't mentioned in the manual, which in fact said not to put in any other game.
- Endless Game: After a while, certain levels repeat themselves and eventually just carries on from Level 1 again. Only if you put in the specific cartridge needed to play the full game, though.
- Exact Words: The Sonic & Knuckles manual states that the game's lock-on technology won't work with non-Sonic games. While it clearly states that you're only supposed to use Sonic 2 or 3 with it, the manual doesn't rule out using Sonic 1 or Sonic Classics 3 in 1 as well.
- Nintendo Hard: Some of the combinations are devilish, especially when running for perfects.
- No Name Given: The original minigame was never given a title. Sonic Mega Collection was what gave it the title that it is known as today. In older collections, it was simply called "Special Stage Mode".
- No Plot? No Problem!: Just press the buttons and go right in.
- One-Hit-Point Wonder: If you touch a red sphere, you instantly lose.
- Password Save: In addition to the password system letting you save your progress through the "full" version of the game, beating the stage given by a non-Sonic 1 or Sonic Classics 3 in 1 cartridge earns you the password for that stage.
- Puzzle Game: Collect the blue spheres and rings while avoiding the red spheres.
- Spell My Name with an "S": Some sources list this game as Blue Spheres.
- Suspiciously Specific Denial: The manual says that putting any game other than the second or third Sonic game won't work. You do it anyway. And then you press all the buttons for no reason. This is further emphasized with the opening screen of Blue Sphere saying "NO WAY! NO WAY! NO WAY! NO WAY?" Notice the question mark.
- Video Game Geography: Topologically, the levels are toroids. Visually, they're spheres. The disconnect can be disorienting, if you notice it while playing.
- Violation of Common Sense: This is a game you play by going against what the manual tells you to do.