It seems this is a ``tough nut'' to crack! A friend of mine calls these sort of questions, where no-one can find a solution for quite a long while, "Mars Bar questions". Perhaps your teacher or your Mum will buy you a Mars Bar if you can solve this. Someone had trouble with this question because they did not know what a plane was and this made us think we should incorporate a maths dictionary on the site. We are working on it!

Here is a hint. Ask yourself ``is there anything special about the angle between the diagonals of a rhombus?'' Now think again about the path of X as the linkage moves.

The following solution comes from Daniel M (age 11) from Northumberland.

"Point X moves around the circumference of a circle of diameter AB. This happens because the four angles at the centre of the rhombus (where the diagonals cross) are always 90 degrees each, no matter where X may be. The angle AXB is always a right angle except when X is at A or at B. The points C, X and A are on top of each other when the path of X goes through A, and similarly D, X and B coincide when X goes through B."