Which Switch is Which? is the second Challenge Act of Chemical Plant Act 1 in the console/PC version of Sonic Generations. It is unlocked after clearing every Stage in the classic era and is playable only as Classic Sonic.
Played as Classic Sonic, this level takes place in a modified version of Chemical Plant Act 1. The player's objective is to simply make it through the given terrain to the Goal Ring. However, several barriers block the player's progress. In order to gradually get rid of these hindrances, the player must find the Switch corresponding to the given barrier in order to get rid of it so that they can move forward.
The Act begins in a modified beginning area, and from there, the player must continue through the stage while activating Switches in order to either open barriers or move platforms. They will move on through the Act and must repeat the process of activating nearby switches in order to progress.