The Rec & Play pens are part of a modern art exhibit that covers sound interaction. The red one records sound on paper, the black one plays it back. It’s no concept, it actually works.
The system operates using a reworking of the old ferromagnetic tape technology used to power the cassette tapes of old, with the recording pen drawing a line of magnetic ink and encoding sound data into it through a built-in mic and tape recording head.
https://gizmodo.com/this-isnt-just-any-old-ironic-misuse-of-a-cassette-tape-5626251
Then your friend plays back the secret message about where to deposit the stolen microfilm using the corresponding black pen, which plays the audio through its little speaker. [Design Boom]
