If you've been following 3D printing in recent years, you may have come across a "next new thing" that sounds like it was dreamed up for the twilight zone: 4D printing, so dubbed and promoted by Skylar Tibbits, director of MIT's Self-Assembly Lab. You unlock this technology, as Rod Serling would say, with the keys of chemistry, physics, engineering and materials science, and move into a realm of both molecular properties and computer-aided design (CAD). Then you cross over into programmable materials -- and that's the term that leads to most of the news made in this field since 2014. Once they're produced on 3D printers, objects made of programmable materials continue to take shape, folding, unfolding or assembling themselves in response to outside stimuli such as light, movement, heat, pressure or water. Tibbits' TED Talk videos demonstrate multimaterial, printed 2D strands that curl themselves into the letters "MIT," and printed flat sheets of programmable materials that, once robotically cut, transform themselves into shoes. Tibbits said 4D printing started as a way to "print" robots, but taking out all the electromechanical systems.