If you've been to the stunning new California Academy of Sciences in San Francisco, you may have noticed a number of interactive exhibits in the halls on the first floor. Among them are two game-like pieces by Snibbe Interactive that allow visitors to physically interact with a projected "natural" environment via motion sensors.

Bug Rug by Snibbe Interactive at the Cal Academy of Sciences, from a video of the installation.
One is called Bug Rug and is set on the floor of a Madagascar forest with insects running around under fallen leaves and branches. Visitors can scare the bugs by stomping around, or they can trap them to learn more about them by guiding bait into traps with a very specific gestural interaction. In the other, Arctic Ice, visitors use their shadows to block the sun's rays, allowing ice to form so that a baby polar bear can find its way back to its mother.
After watching kids play with both, and speaking with someone intimately involved in the installation of the works who's watched people interact with both quite a lot, it's pretty clear that visitors tend to be more engaged and successful with Arctic Ice than with Bug Rug. In pondering why this is the case (beyond the obvious fact that for most people, baby polar bears are a lot more compelling than bugs), I've landed upon the theory that the physical interaction of using one's shadow to block the sun's rays is a lot more natural and discoverable than placing one's hands next to each other palm down, with thumbs touching to move things around on the ground.
With the increasing prevalence of physical and gestural interactivity, from the iPhone to Jeff Han's election night Magic Wall spectacle on CNN, to the Wii, it's likely we're all going to be faced with the excitement and challenge of interacting with and designing devices and environments in new ways. One of the biggest challenges associated with physical interactivity is the lack of transparency into the "commands" or actions available with a given device or environment. The graphical user interface was, in many ways, a huge improvement over the previous idioms of the command line because it made it much more obvious what commands were allowable in a given context. Looking into the brave new future of physical interactivity, we're confronted with the need to create idioms and vocabulary that are as discoverable and useful as possible to avoid stepping back into command line-like arcana.