the entire island for a meal of a different international cuisine. As explained by Mr. F:

“The A.’s run an American restaurant and serve only real American cooking. You are now eating at the B.’s. This is a British chop house. The C.’s run a Chinese restaurant. The D.’s run a Dutch restaurant, the E.’s an Egyptian restaurant; you can run through the alphabet up to T. The T.’s run a Turkish coffee house.”

“And you, Mr. F., run a French restaurant?”

“It’s as easy as that,” said Mr. F.

Mr. F.: This gentleman is one of the 20 patriarchs of the island and lives with his wife, his son F-1, and his daughter F-2, in their French-styled home. He is Sherman’s guide throughout the island.

Mr. M.: The island’s “founder/discoverer.” Mr. M. was shipwrecked on Krakatoa, discovered the diamond mines, built a raft, and made his way back to civilization, where he sold several diamonds in order to finance his utopian vision for Krakatoa. He is the one who gathers the 20 families and organizes their trip.

The main character in the book The Twenty-One Balloons is Professor William Waterman Sherman, a retired math teacher. He leaves San Francisco and intends to fly over the Pacific, but three weeks later, he is rescued by Captain John Simon on the Atlantic with 21 deflated balloons. He tells the story of what happened to him to members of the public and of the Western American Explorers' Club in San Francisco, of which he is an honorary member.

Professor Sherman landed on the volcanic island of Krakatoa, where he was found by "Mr. F." He discovered twenty families living off the proceeds of a diamond mine. In the utopian society that they have arranged, each family's name starts with a different letter of the alphabet, starting with "A." In the A family are Mr. A., Mrs. A., and their son (A-1) and daughter (A-2), and each family is named with the same system. In their Restaurant Government,  they each make food that starts with that letter. For example, the "A" family makes American food. The last family is "T," who run a Turkish restaurant. They have divided the calendar into twenty-day months, and everyone eats at each others' restaurants on one day of the month. They also have houses in which the decorations start with their letter of the alphabet. For example, Mr. F. lives in a house that has a French style of decoration. The island was discovered by Mr. M., who brought a few diamonds to San Francisco to sell and who recruited the other families. Each year, they sell a diamond to pay for their supplies. Eventually, the volcano explodes, and they escape on the balloons with Professor Sherman. They decide to resettle in different places, such as India and Belgium, while Sherman is rescued in the Atlantic. 

The main character of this story is a teacher named Professor William Sherman who takes a balloon trip and lands on a Pacific island called Krakatoa.  The island is home to 20 families who share the wealth of a diamond mine and must keep their treasure a secret.

The 20 families, who live on the island have alphabet names, Family A, etc. The Professor is the only real character in the book.

There is more than one theme present in this book. The majority of the book is about the Professor's time on the island of Krakatoa, its incredible population of people, and the society that they have built. They have more or less built a utopia on a volcanic island, and they have managed to keep it a secret. The people on the island are all incredibly intelligent and hardworking people, and this shows off a theme that emphasizes the importance of education. Additionally, the people are not selfishly using their ingenuity and hard work. The community is exceptional for their altruistic behavior. This book shows young readers a wonderful theme about community and the importance of helping out your neighbors. Creativity and individualism are also emphasized in this book. The island society strongly supports these kinds of attitudes and behavior. A bit of evidence that supports this is the balloon merry-go-round. The entire thing was conceived by the children, and the adults were very helpful in allowing the kids to pursue this particularly creative piece of entertainment.

The Twenty-One Balloons stresses the themes of creativity, individuality, eccentricity, and cooperation. The fantasy of The Twenty-One Balloons is built around an actual historic event—the massive volcanic eruption that destroyed the Pacific island of Krakatoa in 1883. But there the connection with history ends. The Professor discovers that the inhabitants of the island have established a unique, Utopian society, which he seeks to understand. The story begins when Professor Sherman, who teaches high-school, becomes bored with his life and sets off on a journey in a hot air balloon called The Globe.

He hopes the wind will blow him and his balloon all around the world. To his surprise, he instead has a crash landing on the mysterious island of Krakatoa, an island full of diamond mines and enormous wealth.

The secret society of Krakatoa is based on values of greediness for wealth and inactivity. They believe their lives are perfect because they never have to worry about money. They live empty and unfulfilling lives and must learn the value of relationships, education and their own lives. They must learn how having extreme excess of money, or anything, is worthless.

Threatened with destruction, the Professor and the inhabitants must cooperate and discover a way to escape the island before the final explosion.

I think this question is asking about something other than possible themes. It is possible that the author had several messages in mind. One possible message is that the author was trying to show people what a society could be like if everybody worked together and behaved much more altruistically than currently done. It's also possible that the author was trying to send a message about the value of art and science working together, suggesting that they should do so more frequently.

Personally, I feel that the message is squarely aimed at taking mankind's power down a peg or two. The people on the island are amazing. They are friendly and intelligent, and they invent all kinds of amazing technology; however, none of that could prevent the destruction of everything that they built. Escape was possible, but their homes and lives on the island have been destroyed. The message could be something along the lines of "nature always wins," and it shows readers that nature will always be more powerful than humanity.

Pene du Bois published this book in 1947, so long ago that we might have a difficult time understanding the context in which he wrote. One idea that comes to mind is that he was poking fun at Utopian societies in the book, and perhaps making the point that no matter how perfectly we set up societies, nature has a way of making things imperfect. You might also notice that the book was published almost immediately after World War II, so it is equally possible that he longed for a peaceful island on which people could create an ideal and happy society. In either case, a similar eruption really did happen on the island of Krakatoa, and whether the volcano stands as a symbol of the damage to civilization in World War II, or a symbol of nature's way of gumming up the plans of people, is something we may never know.  It is equally possible, given that the author wrote many children's books, that he simply wanted to write a great adventure story, which he most certainly did!

Professor William Waterman Sherman is the main character of the story.  After chapter one, he also becomes the story's narrator.  He is sixty-six years old, and a retired arithmetic teacher, from San Francisco.  In the first year of his retirement, he decides to build and fly a hot air balloon, and hopes to be the first man to fly across the Pacific Ocean.

While there are several smaller conflicts within the greater context of the story, the main conflict occurs when his balloon is punctured by a seagull, and crashes on the island of Krakatoa.  This island is actually an active volcano, which is about to experience a massive (and historically accurate) eruption.

There are several minor characters that Professor Sherman meets while stranded on Krakatoa, who are members of the 20 families that inhabit the place.  These families live in a secret and wealthy utopian society, and each are named for a letter of the alphabet, from A on down.

The Twenty-One Balloons by William Pène due Bois follows the tale of Professor William Waterman Sherman, who retires from his job as a schoolteacher in order to fly across the world by himself in a hot air balloon, planning for the trip to take a year. 

The rising action of the novel starts with the discovery that Professor Sherman has wrecked in the North Atlantic amidst twenty balloons, having completed his circumnavigation in a mere three weeks. Professor Sherman recounts this tale, including the heightened perils he faced when a seagull punctured his balloon and caused him to crash on Krakatoa, a volcanic island, where he discovered a wealthy society whose lifestyle was fueled by a diamond mine. The novel describes the details of this society, which leads up to the climax of the novel—the eruption of the volcano and the escape of the island's population on a platform raised by the twenty balloons.

The climax of this story is when the Utopian people decide to work with the professor to save the lives of everyone on the island, putting human life before their wealth.  Many things happen to lead the plot to this point.

First, the Professor decides to use a hot-air balloon to escape from society and have some solitude.

Then, as irony would have it, he lands on the island of Krakatoa, coming again into contact with society, just not his own.

Next, he interacts with the Krakatoans and learns about their perfect society and its relationship to their excessive wealth. 

Finally, the danger of the volcano becomes apparent, and the villagers and the Professor must rely upon each other if they want to survive the volcano and escape the island safely.

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.

One theme is that equality helps to bred respect.  The members of the island have worked out a system in which all individuals and all families are on equally footing.  Everyone works the same amount of time and everyone makes the same amount of money.  Because their is no disparity, there is no need for resentement, fear or jealousy.  Therefore, all the islanders respect one another, which is demonstrated by the careful rationing of food and failure to argue and fight.

The importance of duty is another theme of the story.  As shown above, all the islanders do their duty accordingly.  They work their allotted time in their restaurant and serve their community members without complaint.  They work together for the common good, and not their own, because they understand that it is their duty to do so under their constitution.  Likewise, the professor - after returning home - resists the appeals of the mayor and the President of the United States to tell the tale of his trip.  Having made an oath to the Explorer's Club, the professor knows it is his duty to tell them first, and hd eagerly abides by that duty. 

There are a few conflicts in this story.  One is man versus nature, first shown as the professor attempts to travel in a hot air balloon and wrecks because of a bird, and then shown as a group of people attempt to live in a civilized way on an actively volcanic island.  Another conflict, resolved by the people on the island, is greed versus cooperation.  As we can see in this story, the "alphabet" families have created a wonderful society that is egalitarian and free of strife.  If it were not for the volcano erupting, the families could still be there, living happily. 

I must say that this is one of my all-time favorite stories, perhaps because it is such a pleasant tale, with human conflicts resolved so nicely. 

The main conflict of a story is the problem that effects the main characters. Other characters are directly or indirectly effected by the conflict, as well, hence the term --external. An internal conflict is a problem that the character faces within himself or herself. For example, if the character has a secret fear or insecurity, that could be an internal conflict.