The solutions eventually become more gymnastic - opening new portals mid-fling and plummeting back through those you've previously opened with pinpoint precision. But by then you're ready, and performing deliciously counter-logical mental inversions at breakneck speed is something to be relished.

The atmosphere, meanwhile, grows thickly sinister. Your sing-song robot guide GLaDOS (you'll find out what it stands for) doesn't seem unduly invested in keeping you alive. Soon her own delusions creep into her instructions to you. "The weighted companion cube," she announces as you snatch up a box, "will not threaten to stab you and cannot, in fact, talk. If the weighted companion cube does talk, the Enrichment Centre urges you to disregard its advice."

But as her coldly voiced lines become more murderous and surreal, they also get funnier. The writing is effortlessly sharp throughout, and with its single inhuman character Portal taps a thick vein of black, absurdist humour that becomes the game's propulsive force. You'll play faster just to hear the next beautifully unhinged line. The game escalates magnificently. The puzzles change nature, requiring you to beat the system with the tricks it taught you rather than jumping through hoops. And at the same time, the humour reaches fever pitch - GLaDOS becomes so brilliantly deranged that at times it's hard to stop yourself from laughing.