Zoo: An excellent place to study the habits of human beings.

Esar’s quip turns the zoo into a mirror: while ostensibly a place to observe animals, it also reveals the spectators. The line suggests that human behavior—curiosity, gawking, judgment, amusement, even cruelty or tenderness—can be studied just as readily as the creatures behind bars. Implicitly, it satirizes the thin boundary people draw between “civilized” humans and “animal” instincts, hinting that social rituals at public attractions expose herd mentality and performative morality. The joke also carries a mild critique of captivity: if the zoo is a lesson in habits, the most telling habits may be those of the captors and onlookers rather than the caged.