This is related to this question:Does the weight of an hourglass change when sands are falling inside?
At Brigham Young University, there is a display consisting of a sealed off liter bottle with a sunken sealed off hourglass. When one turns over the bottle, the hourglass floats as the sands flow down. Eventually, the hourglass reaches some transition, and precipitately changes from floating to sinking.
It has a sign next to it saying that it's for a fishing lure,and that you should not reveal the answer if you get it.
What causes it to change from floating to sinking so quickly?