This is a drawing of a good demonstration that involved one speaker powering another with no electricity other than that provided by the speaker.
If you hook one speaker up directly to another, the speaker spoken into becomes a microphone producing an alternating current matching the frequency of the voice. The alternating current then powers the other speaker.
You can have over 500 feet of wire and the demo still works.
Learning how speaker work is very helpful for 8th graders in understanding how electricity works.