James,
AFAIK all the bubble lights that I have, have some ground glass or similar material glued in the bottom of the liquid vial. In a few of them, there are some loose bits of grit that can get stirred up by the bubbling. In one, the entire disc of grit and glue has come loose and sometimes bubbles form under it and push it up.
In high school chemistry bits of ceramic "boiling chips" were used for similar purpose, to make the boiling start smoothly with small bubbles and not all at once with a big burst that might splash out the contents of a test tube.