To grow or to shrink: A tale of two rubber balloons
  Two identical rubber balloons are partially inflated with air (to different extent) and connected by a hose with a valve. It is found that depending on balloon volumes, when the valve is opened the air will flow either from the larger (fuller) balloon to the smaller (emptier) balloon, or from the smaller balloon to the larger one. The phenomenon is explained in terms of the non-ideal rubber elasticity of balloons. The full phase diagram for the air flow dynamics is constructed. 
Yan Levin and Fernando L. da Silveira