The protagonist of "The Twenty-One Balloons", by William Pene du Bois, is Professor William Waterman Sherman. Sherman is a retired math teacher who has burned out from years of student antics. He is a likable protagonist who decides to take an eccentric journey in a hot air balloon. He says, "In a balloon you can decide only when to start, and usually when to stop. The rest is left up to nature." The professor is no longer constrained by society. Because of this, the antagonist of the story is not a character, but an idea. Sherman rails against the blind acceptance of how things are. He values innovation, so the antagonist would be society's stagnancy.