Basically, a razor blade, a #2 pencil with a graphite center, and a large coil is a foxhole radio, or another way of saying crystal radio. It's a no battery receiver, it get it's energy from radio waves, using as much antenna wire as you can put out. And although I've never made one, I hear they're not hard to make, and people use two liter bottles as a form for the coil. It's great for when no electricity and no battery is available, and it makes a good project for preppers.