You are given a sealed cylinder that is completely filled with water. Floating in the cylinder is an hourglass. The inner diameter of the cylinder is just large enough for the hourglass to pass through it. Initially, the hour glass has run out and it is floating at the top.
You flip the cylinder over, and the hourglass is now at the bottom and the sand starts running. But, instead of floating up to the top, like you might expect, the hourglass remains at the bottom of the cylinder until roughly half the sand has run through it