In a medium where even a cult favorite puzzle game can become a franchise, it’s refreshing how “Portal 2” meets the high expectations set by the original without sticking to the same old routine. This is one of the rare times when a video game sequel expands on a smart idea and creates something truly memorable.

The original “Portal” was a two- to four-hour distraction within Valve’s 2007 compilation, “The Orange Box,” which also introduced the much more popular titles “Half-Life 2: Episode 2” and “Team Fortress 2.” “Portal” was nothing more than an experiment at the time — one with much to build upon.

In the year that brought “Halo 3” and “Call of Duty 4: Modern Warfare” to the masses, “Portal” stood out for its playful tone, clever script and complete lack of guns — well, the kind that shoot bullets, in any case.

“Portal 2” once again finds the player, alone or in the co-op story with a friend, finding their way through mind-bending puzzles with only the Portal Gun at their immediate disposal, which lets the player create a portal entrance and an exit, allowing a shortcut from one side of the room to another or even a distant wall. The simplicity of the controls (create entrance, create exit) and ongoing goal (get to the exit) limits confusion while broadening the potential audience.

The real challenge (and fun) comes from the co-op, which adds difficulty by doubling the number of portals needed for puzzles and requiring action in-sync between two players. You’ll often take a meal break, realize the solution and race back home with your partner to solve it, like you just discovered the cure for cancer. It’s in these “a-ha” moments that “Portal 2” shines.

Co-op in “Portal 2” is as good of a gateway into gaming as any beloved “Mario” title. Rather than clumsily squeezing in a second player into the five-hour single player, Valve designed an entire campaign of equal length around the idea of cooperative play.

The original “Portal” was memorable, above all else, for being surprisingly funny. Those are two words aren’t often associated with video games. “Portal 2” succeeds in providing memorable lines and characters, without relying on old jokes. Instead we have a cast of new characters, including Wheatley, a neurotic, incompetent robot that always says the worst things in the nicest way. “Assuming I find your dead body, I’ll bury you. Go team!” he cheers after you fall down a pit early on.

Fears that “Portal 2” would require extensive knowledge of quantum mechanics should be laid to rest. Valve, developers of “Half-Life” and “Left 4 Dead,” have done an outstanding job of introducing numerous new environmental elements within a gradient that makes every puzzle approachable, overwhelming at first but slowly revealing its secrets through great level design. In fact, an experienced player can run through the single-player without getting stuck on more than a handful of puzzles.

Since 2007, head shots and competitive multiplayer have become ubiquitous, making “Portal 2”’s logic tests, quotable lines, heartwarming conclusion and focus on teamwork all the more memorable. No amount of delays could detract from this timeless experience.