  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.
