Now dodge more missles. It's a bit like reading the end of a book before even starting it. Actually, for a challenge, put in any cartridge made by Electronic Arts. Their special stages are quite hard, especially the FIFA Soccer series. This rules out Shining Force, Soliel, PGA Tour Golf and others.
Passwords are unaffected.
This means you can get to parts of levels that you wouldn't be able to get to with Sonic alone. One downside to this is that Knuckles has a slightly lower jump than Sonic or Tails, which makes some levels harder than they should be, especially Death Egg Zone. The special stages are pretty straightforward. All you have to to is navigate around a giant 3D ball, collecting Blue spheres and avoiding Red ones. Get all the blue spheres and you get an emerald.