Two perspex tubes, filled with water. (In fact a little bit of water has evaporated over the years, so the tubes are not quite full any more). Each tube contains an hourglass; one is floating at the top of the tube, and the other one has sunk to the bottom of the tube. This is a stable state - everything will stay like this indefinitely.
When you turn the whole mechanism upside down, the hourglass that was floating at the top now stays at the bottom of the tube, and the hourglass that was at the bottom is now at the top of the tube. In other words, the hourglass that was previously floating is now at the bottom of the tube, and the hourglass that had sunk to the bottom is now at the top of the tube. How can this be? Are they in fact not freely floating/sinking at all, but fastened to the sides of the tube?
Even more mysterious is what happens next. After a minute or so, the hourglass that is now at the top of the tube sinks back to the bottom, and the hourglass at the bottom of the tube floats up to the top. In other words, each hourglass is now back in its original position.
How does this happen?
Answer on the next page...