When they touch or move objects around things happen objects bump, drag, crash or smush against each other. Paper cannot provide this. Our experimentation with local tools provides convenient solutions to many of the conflicts caused when multiple users access interface elements. The local tools metaphor is very direct, functionality is no longer hidden in menus or dialogue boxes, but instead directly available on the work surface. I believe the local tool metaphor is flexible, powerful, and very closely models how kids actually work.
Lenses provide a way to change how users can interact with objects.
This can be used to allow users to create objects that only they can manipulate, or story elements that only they can see. As argued earlier, Sushi needs to provide a reactive environment. Users need to be able to specify what kinds of events should trigger reactions. The reactions they specify should include playing sounds, making size or color changes, or even animating objects across the surface. I must devise an interface simple enough for the kids to use, while still allowing this type of powerful features.