A book with latches on its pages: each page can lie flat or be folded back. To find out which pages are folded, you don't need to flip through the book. Just tap the cover lightly and listen to how sound travels through the pages. A flat page gives one sound, a folded page another, because the folded page presses against a magnet and starts to vibrate differently.
The scientists built a chain of three such cells and tested all eight possible configurations. Each time they sent a weak pulse from one end and recorded how the last cell shook. The response was different each time, like a signature. Then they built a chain of eight cells and showed that the response can even measure how strong the input tap was. It's as if the book itself told you how hard it was slammed shut.
The same structure can let certain vibration frequencies through and block others, depending on which pages are folded. So these cells could be assembled into mechanical filters and sensors without any electronics. Someday they might become a computer that works on clicks and pushes, not electricity.
The difference in resonance frequencies between states 0 and 1 is 254 Hz, comparable to the difference between the low note of a double bass and the high note of a violin.
The idea of mechanical memory without electronics echoes the computers from Frank Herbert's "Dune," where the ban on thinking machines forced people to create alternative information-processing technologies.