Suppose one holds two sticks in the form of a cross in one hand, and places a rubber band in tension around the four ends. When it is released, it comes to rest in the shape of a convex quadrilateral in a plane. It always returns to the same shape, no matter how it is distorted, as long as the ends do not slip, and the rubber band does not break. This is a very simple example of a "rigid" frame...
