Building a World War II Foxhole Radio
When I was a kid, I was fascinated by crystal radios. The first one I had was in the form of a backpack for the Six Million Dollar Man action figure ( Mom picked up the crystal set at some kind of sale before I ever had the action figure ).
The thing that fascinated me about crystal radios was that they required no external power-source; the radio signals themselves were strong enough to power a small earpiece at an acceptable volume.