You may already know that "Portal" was inspired by a student game made out of Digipen called "Narbacular Drop." One of the students that worked on the game, Kim Swift, went on to join the team at Valve as a lead designer on "Portal." In the years since, Swift left Valve and joined the crew at Airtight Games to work on a new project that's about as mind-bending as her best-known work. That project is "Quantum Conundrum."
The two games are similar in that they're both in the puzzle genre and played from the first-person perspective. They also use a single gameplay element as the crux of all of the puzzles. In the case of "Portal," that element was, well, portals. In "Quantum Conundrum" it's all about changing dimensions.
"Quantum Conundrum" gives you the power to change between several different dimensions at will in order to solve a wide range of puzzles. One of the early puzzles involves moving a heavy couch to create a step, allowing you to access a higher platform. In our normal dimension, the couch is far too heavy to be lifted, but change to the Fluffy Dimension (yes, that's seriously what it's called) and the entire world and all of the objects in it become the consistency of a stuffed animal. So the couch becomes 10 times lighter, letting you move it with ease before turning back to the normal dimension to use it as a sturdy step up.
Conceptually it seems pretty simple, but the game quickly introduces new variables, like object-destroying lasers and new dimensions like the Slow Motion Dimension, where objects move at 20 times slower speeds. It's easy to imagine how these elements could quickly stack upon one another, making a truly devious puzzle game.
Gamespot has an exclusive walkthrough of the game, which you can watch above. It's much easier to grasp the concepts at work when seeing them in motion, and it also gives you a sense of the art design for the game, which ditches the cold metallic realm "Portal" in favor of a more colorful, family-friendly world. Smart idea. The kids won't even know what hit 'em.