Let's be clear about one thing: 2D Boy's World of Goo HD is one of the best puzzle games for the iPad. With a recent update that makes this version of the app run on the iPhone 3GS, iPhone 4 and third- and fourth-generation iPod touches, it may be one of the best puzzle games in the App Store.
The plot in World of Goo HD may be nonsensical, but the levels all follow the same format. At the start of each level, you should see small goo balls sitting dormant at the bottom of your screen. Elsewhere on your screen, usually up high, you should see an open pipe. Between the pipe and the balls should be a complicated maze/structure of some kind. Using your finger, you have to stack the balls up to reach the pipe. Once you get your goo structure to reach the pipe, the pipe will suck up all the goo balls not used to build the structure.
Each level lets you know how many balls need to be sucked up the pipe for you to successfully complete the level. So when you build a structure, you have to do so keeping a certain number of goo balls free for the pipe.
The beauty of World of Goo HD is that things never get repetitive - no level feels like the last. In one level, you have to build a tower of goo inside an animal's stomach so that the level's goo balls can reach a pipe at the base of the animal's mouth. In another, you have to build a goo bridge that can stretch across a canyon. In yet another, you have to build a goo structure that can encapsulate a windmill.
World of Goo levels are each a challenge and take a great deal of thought to complete. Just when you think you have built the perfect structure, you might find that you don't have enough goo balls for the pipe. Or, just when you think your structure is about to reach the pipe, you might find that it collapses on you. There's a lot of strategy involved when you play the game.
Graphically, the game soars. Its levels are littered with interesting backdrops, involving canyons, water fountains and volcanoes. Levels also tend to have cool-looking machines and animals as a part of them. Level artifacts are usually well coloured - rich with greens, whites, blues and blacks.
Though World of Goo HD has great visuals, its sounds can get annoying. And despite extensive searching, I couldn't find a button in the game's interface that allows me to turn off its sounds. I recommend you play the game with the volume turned way down on your iOS device.