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The iPhone's touch control and accelerometer (which registers the change in orientation when you tilt the phone from horizontal to vertical) enable it to support all sorts of innovative game designs. One of the coolest I've seen so far is Hudson Software's Aqua Forest, which is part puzzle game and part free-form creative tool.

Splash Hit: The goal in this Aqua Forest puzzle is to fill a cup with water by plugging a stream.

Using a unique physics engine called OctaveEngine Casual developed by a company called Prometech, Aqua Forest mimics how water, solids, and gases behave and simulates the actions of objects with different physical characteristics. And with a built-in set of painting tools, you can create everything from a pool of water that sloshes around your iPhone screen depending on what direction you're holding it, to a receptacle to hold that water, or a hot substance that causes that water to heat up and turn to vapor, or a cold object that causes that vapor to condense back to water. It's thoroughly amazing, and that alone--available to you in a Sandbox mode called Free--is worth the $8. (You can also save and load files you're working on.)

But Aqua Forest is a game, too. The developers have included five different puzzle modes--Tilting, Touch, Drawing, Warm And Cool, and All Functions--each with 10 games. There's no scoring or beating the clock here--they're untimed puzzles that encourage you to figure out the physics to solve them. At first you simply have to tilt the screen to win--moving a ball around a maze, or moving fluid from one container to another. Eventually, though, you'll have to use all your skill to figure out what to do, and that makes for some challenging fun. There is a point to all this, too--each time you complete a set of puzzles, you get a surprise. I won't spoil it, but here's a hint: the game's name is Aqua Forest.

Aqua Forest has a few quirks--the physics simulation, while remarkable, is positively sluggish at times. And while the developer paid careful attention to the puzzles, it seems to be at the expense of the interface--some of the language is awkward, and navigating through the puzzle menu is difficult. For example, you can expand each category, but there doesn't seem to be a way to collapse it. My attempts to reach the developer for tech support met with silence for several days, followed by a form letter. I hope the level of support improves.

Based on the user feedback on the App Store, it seems that many early buyers have overlooked the built-in documentation. (There's a Help button on the main screen.)

Still, the non-violent gameplay, gentle sound effects and new-age style music and sandbox mode make for a compelling and unique iPhone and iPod touch experience, and a charming and delightful diversion.

Aqua Forest is compatible with any iPhone and iPod touch running the iPhone 2.0 software update.

[Peter Cohen is a Macworld senior editor and Game Room columnist.]