This Demonstration is a simulation of a very simple "cat"—a deformable body that can change its orientation using only internal forces, enabling it to rotate while its total angular momentum is zero. This model was proposed by J. E. Avron and was studied in detail in E. Putterman and O. Raz, "The Square Cat," Am. J. Phys. 76
(11), 2008 pp. 1040–1044. The four masses are connected with rods of zero mass. By applying internal forces only, the user can control the length of one pair of rods
and the base angle
of the parallelogram. Starting from a given shape, changing
to its original value, and then changing
to its original value, also (all changes are done while keeping the angular momentum of the body zero), leads to a net rotation of the original shape.