The cassette you’re holding contains an invention humanity has used since 1928. Inside is a strip of polyester plastic tape, 135 meters long in a standard 90-minute cassette, only 3.8 millimeters wide. The tape is coated with a thin layer of iron oxide, a material that retains the magnetic field it’s exposed to. That’s all you really need to know: music inside a cassette is simply an arrangement of microscopic magnetic particles.

When you press RECORD, a tiny electromagnet inside the cassette player, called the recording head, receives an electrical signal that mirrors the sound. The head generates a magnetic field that aligns the particles on the tape into a specific pattern. The tape moves past the head at an exact speed: 4.76 centimeters per second. That’s it. That’s all that happens.

When you press PLAY, the process reverses. The tape passes in front of a playback head that reads the alignment of the particles and converts it back into an electrical signal, then into sound.

The magic is that you can press RECORD on any cassette, anytime. You can erase what’s already there and replace it with something else. The cassette doesn’t judge you. It accepts whatever you give it and preserves exactly what you asked it to keep. That’s what made it the most democratic audio technology in history.