A great game involving solving puzzles using two linked gates called "portals." If an atom, photon, or any other form of existance enters a portal, it comes out the other corresponding portal. Portals are designated by a blue or orange lining, shaped in an oval similar to the logo. Portals cannot be created through portals. At the beginning, the player has no control of portals, but later gains a device capable of creating blue portals, and later both portals. The main character is mute, and there is (arguably) only one character you interact with in the game. Despite this, the game is throught-provoking and the lines are often humorous.