Ship!!!

Place a half-dollar or 10 new pence on the table and on one side of the coin a small cork disk. How can you move the cork to the exact centre of the coin without touching it!

Pour water on to the coin - drop wise, so that it does not spill over - to form a water mountain over the surface. At first the force of gravity holds the cork on the edge of the slightly curved water surface. If you now pour on more water, the pressure of the water on the edge increases, while it remains constant on the top. So the cork moves up the hill to the middle, which is the region of lowest pressure.