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.
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]