Well, we know the incoming waves send electricity back and forth in the coil. But we know we must fix this electricity so that its changes will go in only one direction into our earphones.
The safety razor blade and the piece of lead act like an electrical gate. The blade is made of steel and the pencil point is made of carbon. When the steel and carbon touch each other, they will let electricity go through in only one direction. Once the electricity gets through the razor blade and the safety pin, it goes into our earphones.
But one of the most interesting things about radio is that if we can do something one way, we often can do it another way.