Use a white noise generator or a radio tuned to a place between stations to send a signal to the amplifier and then the speakers. Place the two speaker facing each other. Static noise should be clearly heard until the speaker phase is reversed producing destructive interference. Move the speaker with the long cord away from the other speaker and listen as you "pour" the sound out.  Return the speaker to its original place and the sound goes away once more. This demo is especially nice because the incoherence eliminates interference effects in the room and you can readily hear the impact on the bass response.

Place the two speakers close to each other and play an 80 Hz. tone into both. Switch the phase and show that the sound almost disappears. Move one of the speakers until it is about 15 feet away from the other and the bass tone will return showing that the two speakers are in phase again.