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"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."