"A Little Boy By the Name of Billy" joke

There was a little boy by the name of Billy. Billy was an ordinary little boy who did ordinary little boy things, like playing, eating, bathing, destroying things, and going to school. One day, when Billy went down to the bus stop to meet the bus to go to school, he found all of his friends huddled around in a little group, talking about the Purple Wombat.
Being a little boy, Billy was curious. So he asked them, "What's the Purple Wombat?"
"You don't know what the Purple Wombat is?" the children exclaimed disgustedly. For the rest of the morning, they would not go near Billy, always standing far away and staring at him. Then the bus came. Billy, confused, got on the bus along with the rest of the children.
"Hey, Mister Bus Driver!" one of the chldren shouted. "Billy doesn't know what the Purple Wombat is!"
The bus driver turned around abruptly. "You don't know what the Purple Wombat is?" he said in disbelief. He ordered Billy to sit in the very back of the bus, all by himself.
Eventually, they got to school, and Billy got off the bus and went to class. Class proceeded normally; the students did the pledge of allegiance and worked on their multiplication tables for a while. Then the teacher led them into a unit on geography. Billy was not really paying attention, but he heard the teacher mention something about the Purple Wombat.
Billy's hand shot up, and, when the teacher called on him, Billy asked, "Teacher, what's the Purple Wombat?"
"You don't know what the Purple Wombat is?" the teacher cried in alarm, "Get yourself to the principal's office right now, young man. No, no buts - march!"
So Billy headed down the long, dark, frightening hallway to the principal's office. He slowly opened the large, heavy door, and timidly entered the room behind it. There, at a large, imposing desk, sat the principal. The principal was a hulking man, balding, with a thin mustache. He spoke in a deep baritone voice. He was enough to frighten little boys like Billy who had been sent to his office almost to tears.
"Well, Billy," he began slowly. "What seems to be the problem?"
"Mr. Principal, I just don't know what's going on today. Everyone's been acting weird, and they're all treating me really badly. Like teacher just sent me to you and stuff."
"Now, Billy, I'm here to help you. I'm the princi-Pal, after all. Heh heh. Can you tell me why everyone's acting so strangely?"
"It's because I don't know what some stupid Purple Wombat is."
"What? You don't know what the Purple Wombat is? That's it. I am calling your mother, young man. Consider yourself suspended."
The principal threw Billy out of his office and told him to go home. Billy, crying, began the long walk home. When he got there, his mother was standing in the doorway waiting for him.
"Billy!" she called, sobbing, "I was so worried about you! What happened?"
"Mom," Billy cried, "Everyone was being mean to me and I had to sit in the back of the bus all by myself and the teacher sent me to the principal's office and the principal suspended me, all because I don't know what the Purple Wombat is!"
"What? You don't know what the Purple Wombat is?" Billy's mother shrieked. "Go to your room this minute. Go! Just wait until your father gets home!"
So Billy marched up the stairs and into his room. He collapsed on the bed, crying. After some amount of time, he heard a car pull in and some doors shutting. His father was home. He could hear his parents talking downstairs but didn't know what they were saying. Then he heard footsteps coming up the stairs, and his door opened.
"Billy," his father began in that lecturing-father tone, "Your mother says you've been acting badly lately. Would you like to tell me what you've done?"
"Dad, I haven't done anything! I just don't know what the Purple Wombat is!"
"You...don't know what the Purple Wombat is. Well, in that case, you can just stay in this room all night, mister. And forget about dinner!"