I will present some unusual computer user interfaces in which the sense of physical manipulation is pushed to an extreme. One such system, the Alternate Reality Kit (ARK), is a kind of animated interactive world for building simulations. In ARK, everything appears as a physical object with mass and velocity, including normally intangible things such as a compiler or a law of nature. Thus, the user can grab gravity itself, carry it, throw it, or modify it by attaching buttons and sliders. The peculiar combination of concreteness and abstraction made possible by computer-based realities has some interesting ramifications. Examples to be discussed include the issues of use vs. mention and reflection in the user interface. The talk will feature lots of fun videotape, including bits on a multi-user "shared space" version of ARK, and a version of the system in which objects make sound.