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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?

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Very interesting demo. Would this invalidate the top answer to your linked question? –  BMS Dec 15 '13 at 20:21
    
@BMS Based on fibonatic's answer it doesn't - this setup just seems to be designed for the sole purpose of confusing physics students with regard to the original hourglass question. –  Chris White Dec 16 '13 at 5:52
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I assume that you mean this: Floating Hourglass Illusions

And it has a very nice explanation at the end of the article:

One hourglass is slightly positively buoyant, and the other one is slightly negatively buoyant. So the starting position is that one glass is at the top of its tube, and the other is at the bottom. However when you turn the device upside down, each inverted hourglass now has sand at the top, and air at the bottom. This makes it top heavy, or bottom buoyant if you like, and it has a tendency to try and flip over. However it cannot do this because it fits fairly snugly within the tube. But the effect is that it wedges itself in, and it is held in place by friction. Technically this is static friction, which is sometimes called 'stiction'. As the sand falls through the hourglass, its tendency to flip over is reduced, until it 'unsticks' from the side, and positively buoyant glass floats to the top, and the other descends to the bottom. The trick depends on the two hourglasses being only slightly positively or negatively buoyant. Were this not the case, their natural buoyancy would be strong enough to overcome the 'stiction' effect immediately, and the trick would not work.

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A variation in this video youtube.com/watch?v=kctdo6rQZbY –  anna v Dec 16 '13 at 5:02
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