tampon-transponder

a covert communication device inserted in the vagina of a secret-agent or sexy spy. The earliest model, developed by the Brazilians, could only send an out-going message encoded by the secret-agent squeezing her vagina in Morse-code, which was then converted the Kegal contractions into radio signals. The advantage of the device was that it was unlikely to be found even after a thorough frisking or even a strip-search. In the event it was found, the secret-agent could merely explain that she was having her period (as the device looked like an ordinary tampon with the string serving as the antenna). The Ukrainians created an improved version of the device that could receive and convey incoming messages as well. The incoming message, also in Morse-code, was translated into vibrations that corresponded to the dots and dashes. While effective as a communication device, the Ukrainian device suffered from the draw-back that it tended to cause intense orgasms in the secret-agents and, as the tampon could not absorb all the natural orgasmic lubricant due to its being filled with electronic gadgetry, the orgasmic fluid would often carry the tampon-transponder out into the panties of the secret agent, and she would have improvise a hasty trip to the lady's room to reinsert the device before she could resume communication.

Theoretical situation where the biological sex of a person can't be immediatly discerned by direct observation.