Movie S1.

Two-balloon experiment. (i) Two identical balloons, inflated to different diameters, are connected to freely exchange air. Surprisingly, instead of the balloons becoming equal in size, for most cases the smaller balloon becomes even smaller and the balloon with the larger diameter further increases in volume. This unexpected behavior is the result of the nonlinear relation between the balloons’ pressure and volume. (ii) By changing the initial volume of the two balloons, it is also possible to realize a system in which the balloons become equal in size when connected.