Like sand through an hourglass...

[AlanMiller]

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

[HansV]

<span style="background-color: #FFFF00; color: #000000; font-weight: bold"><font color=yellow>Friction? When the cylinder is turned over, the heavy part of the hourglass containing the sand is at the top. This makes the hourglass list (just a little bit, since there is hardly any spare room), and it gets stuck where it is. By the time half the sand has run to the bottom, the two halves of the hourglass have equal weight, after that, the lower half is heavier, so the hourglass will stop listing and get unstuck.</font color=yellow></span hi>

[AlanMiller]

Brilliant! And you beat the clock by 568 grains of sand <img src=/S/grin.gif border=0 alt=grin width=15 height=15>. Full explanation is here.

Alan