Valentine Experiment: Floating Hearts

Valentine's Day means it's time for the mysteriously floating conversation hearts. This year I used Brach's Conversation Hearts.

Drop them in water, and they sink.

After an about an hour, they start to rise.

After two hours, they should all be bobbing at the surface.

Remove them from water (they're a little gooey), drop them back in, and they sink back to the bottom.

I'd love to know more about why they do this.  My guess is that enough air bubbles collect on the hearts to make them float, but more research is definitely called for.