There is standing a flat cylinder at its side with a counterweight at the bottom in a fluting. On top a tube is filling a chamber with water. At least the water dislocate the balance point. This forces the cylinder to roll on rails to the left or to the right. After some degrees the water pours out and the changed balance point force it to the old position like a skip jack. I won 2007 a competition with it and finely three of them in a row were built for Singapore Marina Barrage in the dimension of 2m height.