When he had first seen a zoo, Mike had been much upset; Jill had been forced to order him to wait and grok, as be had been about to take immediate action to free all the animals.  He had conceded presently, under her arguments - that most of these animals could not stay alive free in the climate and environment where he proposed to turn them loose, that a zoo was a nest... of a sort.  He had followed this first experience with many hours of withdrawal, after which he never again threatened to remove all the bars and glass and grills.  He explained to Jill that the bars were to keep people out at least as much as to keep the animals in, which he had failed to grok at first.  After that Mike never missed a zoo wherever they went.