I was thinking about the concept of Utility Fog, programmable matter that can replicate a physical structure. They can be made to form air, furniture, and even entire buildings, with sufficient programming.
Given the case that they are mass produced to be able to create relatively anything, like the stuff I mentioned above, the next issue would be about how they can be controlled. Looking at the description from the site above, it seems that people have a direct interface with the Utility Fog, as if the Utility Fog was mind controlled, probably with a brain cap that allows for such an interface. So, when people think they want to have a couch rather than just a chair, the Utility Fog will respond to their thoughts and make just that.
However, I have realised a problem, that does not seem to be addressed. If people no longer actively think about what they want the Utility Fog to be, or turn off the brain cap, won't the stuff just collapse back into a bunch of nanomachines in the air? So, I wonder, what sort of mechanism or programming might help to solve this issue, keeping the furniture in place even when the person does not actively think about them.