The first-person puzzle game that would go on to inspire Portal 2, Tag puts you behind the barrel of a spray paint gun with very unusual abilities. Depending on which color you use, the player can bounce off, speed over, or stick to the level’s surfaces. Combine paint powers to platform your way to the end of each mind-bending stage.
The physics-based puzzle elements of Tag that would end up in Portal 2 actually weren’t present at all in the student team’s initial conception of the game. In fact, Tag was originally designed to be a multiplayer, team-based first-person shooter where players could “tag” opponents and change their color.