Sonic the Hedgehog 2 is a 2D, side-scrolling platform game. At the game's start, the player can select to either play as Sonic, Tails, or both. In the latter mode, players control Sonic while Tails runs along beside him. A second player can join in at any time and control Tails separately. The game takes place over a series of levels, each divided into one, two, or three acts with a boss fight with Robotnik at the end of the last act. The character can jump on enemies to defeat them; the game also introduces a new move, the "spin dash" or "Super Dash Attack", by which the player curls in a ball and spins while stationary, resulting in a speed boost. When the player is attacked by an enemy without rings, is crushed, falls off-screen, or exceeds the act's ten-minute limit, they lose a life and return to the most recently passed checkpoint. Dying with zero lives gives the player a game over. The game's special stages, in which the player collects rings in hopes of obtaining a Chaos Emerald, are presented in 3D, unlike the rest of the game. When the player collects at least 50 rings and passes a checkpoint, they can warp to a "special stage". In these stages, the player runs through a pseudo-3D half-pipe course, collecting rings and dodging bombs. A set amount of rings must be collected to pass through each of three checkpoints and in turn to obtain the emerald itself. If Sonic collides with a bomb, he loses ten rings and is immobilized momentarily. The stages rise in difficulty, and the player cannot enter any stage without passing the previous one. After finishing, the player is transported back to the star post they used to enter the special stage. When all Emeralds have been collected, if the player chose Sonic, he has the ability to turn into Super Sonic, which requires 50 rings or more. Super Sonic is yellow and invincible to enemy attacks, and his speed, acceleration, and jump height are increased as well. However, he loses one ring per second and reverts to being regular Sonic when all of his rings are gone. The game also has a competitive mode, where two players compete against each other to the finish line, as either Sonic or Tails, in a split-screen race through three of the regular levels and a special stage. After one player finishes one of the regular levels, the other player must finish the zone within 60 seconds, or the level ends instantly. In the regular levels, players are ranked in five areas (score, time, rings held at the end of the level, total rings collected, and number of item boxes broken). The player with wins in the most number of categories, wins the level. In the Special Stage, players compete to obtain the most rings. The mode ends when all stages have been completed, or if a player loses all their lives, in which their opponent will automatically win.