The falling slinky
  The slinky, released from rest hanging under its own weight, falls in a peculiar manner. The bottom stays at rest until a wave hits it from above. Two cases -- one unphysical one where the slinky is able to pass through itself, and the other where the coils of the slinky collide creating a shock wave travelling down the slinky -- are analysed. In the former case, the bottom begins to move much later than in the latter. 
W. G. Unruh
physics.class-ph physics.pop-ph