Problem
You left your theremin at home and are lacking a suitable replacement for tonight’s big party.
Solution
Place two or more cell phones in close proximity to each other, make calls to each other (works best if the phones have a speakerphone mode).
Result
Lots of distorted feedback and odd sound effects. Digital cellular phones compress and distort audio (there is also a noticeable transmission delay). Notice what happens when one handset picks up a sudden noise. It’ll emerge, slightly garbled from the other handsets a fraction of a second later, and this will continue for several more cycles until the original sound is unrecognizable.
For extra fun… try this with three or more phones. Also try with pairs of phones using different combinations of cellular carriers. In the USA, there are several different cellular systems, each of which handle audio slightly differently (different transmission delay characteristics, different compression algorithms).
Lacking a pair of cell phones...
I decided to try this with our cell phone and a regular land line. I rang up my Nokia 3650, put it into speakerphone mode, and suddenly an eerie oscillating noise emerged from the phone, waking the neighbors and scaring the cat. Indeed, much like a theremin, you can alter the noise by waving one hand around the other. The best results I got were by quickly waving the cell phone back and forth in front of the telephone handset. The feedback produced this way can make more interesting noises than a simple high-pitched feedback whine, and if you continue to hold the two phones together, the feedback loop keeps the novel sounds going for quite a bit before they degenerate back into the whine again...