I was drinking a bootle of water, while listening to a group of people having a debate. As the water level sunk, I noticed something. In the beginning the bottle would resonate/vibrate when the people with higher pitch voices talked. When the bottle was nearly empty, it would resonate/vibrate (and more so) when people with bassier voices talked.
Using bottles of water (with different waterlevels) and a vibration sensor on each, one could make a low tech thing that detects whether a male or female is speaking in the room. Why not use a microphone and a spectrum analyzer? Well, because you are McGyver and you're caught in some prison, and mission dictates that you find out when the guy with the really bassy voice enters the room. You know, ad hoc spy stuff.
Here is a figure that illustrates the "Low Tech Male/Female Voice Detection System":