In elementary school, I used to listen to the sounds of people’s pencils scratching on the desk, and could accurately determine what they were writing, especially if my head was near the desk and I knew what their handwriting looked like. Curves and lines, and strokes of different lengths and directions against the grain of the paper, have different frequency profiles; lifting and setting the pencil back down generates a tapping sound…that sort of thing.
People often don’t believe me when I tell them this, because they figure that human hearing just isn’t that sensitive, or that the sound of handwriting doesn’t convey that much information—but it does! You’ve simply never noticed or paid attention to it because you had no desire or reason to. Another animal might, for various reasons.
Once, my dad picked up my Magna-Doodle board as we stood outside my room, and started writing a message to me. I listened closely and said “…Clean your room?”—he had written “Cʟᴇᴀɴ Yᴏᴜʀ Rᴏᴏᴍ”—and I’ll never forget his perplexed look, first at me, then turning the board around to see if any light could be shining through. He still didn’t quite believe that I could do what I had just done in front of him!