Looks like the mechanical engineers trumped the EEs this time...
In the summer of 1965 while I was a student at Georgia Tech in Atlanta, I was living with several fraternity brothers near the Tech campus. One of our roommates, an electrical engineer major, had spent several weeks building an AM/FM stereo from a kit on the dining table. When he finally got the stereo working, he hooked it up to his big two cabinet speakers and was testing it by listening to a local FM radio station. After enjoying the music for a while, he discovered that he was missing something and left the house to go to Radio Shack and left the stereo playing.
We, Civil Engineering students and knowing nothing about electronics, decided to play a prank on him. While he was gone, we took the back off of one of the big cabinet speakers and put a small portable FM radio inside and replaced the back. The portable radio was tuned to the same FM station as his stereo and at about the same volume.
Later, when he came back and turned off the stereo to work on it, the one speaker kept on speaking. He stood there speechless, in his head he must have been going over all the technical possible causes of the speaker continuing to play. Thinking that the stereo must still be getting power, he immediately unplugged the power cord from the wall outlet but was surprised that the speaker kept speaking. Next he disconnected the speaker wires from the back of his stereo thinking this would surely fix the problem but the speaker still kept speaking. He then must have begun to conjecture about how some radio signal being induced into the speaker wires causing it to play. So he disconnected the speaker wires from the back of the speaker and was completely baffled when it kept speaking.
We knew the game was finally over when he went to his tool box and returned with a Phillips screw driver and began removing the back of the speaker. He was a little mad when found the portable radio blaring inside and knew that he had been the victim of our simple prank.