The 3D-printed watch, less than 3 inches in length, resembles the original full-size machine. (It even includes miniature, nonfunctional floppy disks.) But it’s much more powerful. A Teensy 3.1 ARM processor runs at 72 megahertz and has 64 kilobytes of random-access memory (RAM). The screen is a 1.8-inch liquid-crystal display—inconceivable in the late ’70s. The user interface, though, is simple: a knob that winds through several parody apps, including a clock that displays an arbitrary time, a fitness tracker with progress bars that change randomly, and a weather app—a green, low-resolution picture of Earth.