The physics of articulated toys - a jumping and rotating kangaroo
  We describe the physics of an articulated toy with an internal source of energy provided by a spiral spring. The toy is a funny low cost kangaroo which jumps and rotates. The study consists of a mechanical and a thermodynamical analysis which makes use of the Newton and center of mass equations, the rotational equations and the first law of thermodynamics. This amazing toy provides a nice demonstrative example how new physics insights can be brought about when links with thermodynamics are established in the study of mechanical systems. 
J. G\"u\'emez, M. Fiolhais
physics.class-ph physics.pop-ph