I'm contemplating making a list, a bullet-pointed account of the defining moments in the 18-odd years I've been playing video games. Rewinding time in Prince of Persia after landing on a bed of spikes would be on it for sure. So too would that moment in Metal Gear Solid where I had to read the back of the game box in REAL LIFE to learn Meryl's codec. A more recent addition might be that bit in Heavy Rain where I was forced to saw off my own finger. Also on this list that I'll probably never get round to writing - and fairly high up on it, I'd like to point out - is the first time I emerged from a portal in Valve's game of the same name. Even after the hundredth time, the physics behind this fascinated me. It was a mechanic that refused to lose its novelty, and yet at the same time it was like riding a bike: once you've learnt how to use it, you're not likely to forget. Co-op in Portal 2 changes all that.

Getting on this new bike, you'll realise you're going to have to teach yourself a fresh way of riding; where your two pedals used to be, there are now four. Instead of having to worry about just two portals, you'll now have to get your head around four. While the action still plays out exactly as you would expect it to, it takes a little while to re-wire your brain to this new way of thinking. Thankfully, the test chambers I recently sampled were early on in the game.

The stars of co-op mode are two robots; what Laurel and Hardy might look like if they were redesigned by Aperture Science. Furnished with portal guns, the pair are tasked with thinking their way through a number of test chambers under the watchful eye of another artificial intelligence, GLaDOS. She wasn't lying you know - she is indeed still alive. You'd think she'd be more welcoming of robots - her own kind - but her sharp tongue and insulting nature are just as fierce as ever. Rest assured, the script is just as hilarious and well written as it was in the first game.

The first few puzzles were easy; a simple case of moving cubes onto switches and pressing buttons to open doors. The first mentally strenuous puzzle involved one of those paradoxical infinite loops that portal guns are capable of creating. We were in a room, with the door to the next chamber on the other side of a ten foot gap in the floor. My partner (a Valve employee, incidentally, so he knew what he was doing) fired a portal on the upside of the ceiling behind me, and then popped another one on the floor below it. I jumped in and began falling through a never ending tunnel, gradually increasing in speed as I did so. Once I had reached an appropriate velocity, my partner fired another portal on a wall at the top of the room, and as I fell through the original bottom portal (the top one was the one that was moved), I came hurtling out of the new one and found myself on the other side of the room. From this new position, I repeated the process for my partner and we advanced to the next chamber.

It's not just the fact that there are four portals that makes things more difficult this time around. You can no longer rely solely on your own ability as a gamer. Many players in co-op games get a free ride, carried to the end of a level by the hard work of the other. This doesn't happen in Portal 2. Puzzles are designed so that each player is as instrumental to progression as the other. Communication is therefore vital for success, something made difficult if you're playing online and your headset is lost/broken/digesting in the stomach of a household pet. To avoid this predicament, Valve has implemented a marker system, an ingenious method of telling your partner what do without having to make a sound. By clicking in the left thumb-stick, players can advise their partner where to place a portal by tagging certain items in the game world. While I had the luxury of discussing each puzzle with the chap sat next to me, this tagging mechanic will prove invaluable when I undoubtedly play over Xbox LIVE after its release.

Completing a test chamber in Portal has always been an incredibly satisfying affair. When you nail a puzzle, and the door to the next test chamber slides open, a swarm of endorphins are pumped into your veins, and you get that warm buzz of satisfaction. With the introduction of co-op, you're naturally going to want to share this buzz with your partner. High fives, hugs and bro-fists (for us lads) make the experience all the more enjoyable. As I pointed out earlier, though, many people will be playing online, and thus such physical celebrations of intellectual prowess are sadly not possible. Valve has a work around for this, though. It's included a feature that allows robo-Laurel and robo-Hardy to interact with a range of amusing gestures. I only got to see a wave and a high-five, but it's this injection of character to otherwise sterile objects that gives the game such a unique charm. Amusingly, GlaDOS doesn't like these displays of exuberance, and if they're caught on one of the many cameras in the game, she'll kick up a fuss.

Of the several games I saw at EA's Spring showcase event, Portal 2 left the biggest impression. The demo was over in less than fifteen minutes, but that short amount of time was brimming with fresh ideas and laugh-out-loud moments. While I am of course looking forward to the 'main' single-player event, I think I'm now more excited for the co-op. With twice as many portals, the scope for well-designed and (hopefully) fantastically difficult puzzles is increased, too. I dread to think what some of the end-game challenges will be like, but I genuinely can't wait to tackle them. With a friend helping me, of course.

Portal 2 will be available for Xbox 360, PlayStation 3 and PC on April 22, 2011.