We're not entirely sure how it began, but we know who started it. At some point, perhaps while eating, David's teeth rubbed together just right, producing a slight vibration and a distinctive noise. Amused by this, he worked to reproduce the noise to the point that he could produce it at will. And David willed it a lot.
Julia, amused by David's newfound ability and contagious laughter, somehow picked up on this trick rather quickly and became as adept at this trick.
What follows are periods of teeth grinding followed by laughter. It starts out as each child taking turns grinding teeth, but it eventually turns into a grind-fest where both produce the horrible sound in two slightly different pitches. Perhaps I should play some "Dueling Banjos" to accompany this display...
While watching them go at it tends to be amusing, it's a bit worrisome because of what they might be doing to their teeth. Hopefully it's a short-lived thing that will go away if we don't pay much attention to it. However, since both children find it amusing and egg on each other with hysterical laughter, it might take a bit longer than we'd like.