You might already think World of Goo sounds challenging, but you haven't heard the half of it yet. The basic black Goo blobs are only the beginning. Within the first hour the game throws balloon-like red Goo at you, not to mention a green variety that can be attached and reattached at will. Soon after that you'll get a hideous grey ‘snot' that can be strung out in tough vertical or horizontal lines. Of course, the game isn't out to make things easier for you and your new-found friends. Instead, it finds more and more ingenious ways of making life tricky, with genuinely brilliant levels that tax your grey matter, your guts and your ability to think around a given situation.
Appropriately, World of Goo is the very opposite of a dry, logical puzzle game. If you can't take risks, move quickly or use your imagination, then you're not going to get very far at all. In this respect, it's much like Lemmings or Portal, and just as in those classics it's one thing to work out a solution to the level, and quite another to execute it without making a total mess of the whole thing.
Trust me - there's little more dispiriting than watching a hard put together Goo structure swing, stretch, snap and tumble before your eyes but, unlike some other puzzle games, World of Goo always keeps you coming back for more. For one thing, it's never too difficult. Even early on you'll meet some tricky levels, and the designs get ever more complex and more tricky as you progress, but there's always that feeling that success is only just out of reach; if you could only have that flash of insight or keep that damn Goo from falling apart for one more second, you just know you could have this level licked.
What's more, while there's no mid-level saving, 2D Boy has incorporated a neat quick-rewind button, where blasting one of the small number of tiny fireflies you'll find around the level snaps you back to the time before your last, disastrous action. You can't afford to waste them, but they give a little leeway for error.