"The Purple Wombat" joke

Once upon a time, there was a little boy named Billy. Billy was pretty much your average kid. But one day, he got on the school bus and heard a bunch of kids talking about the Purple Wombat. He asked them what the Purple Wombat was, they got angry at him and sat somewhere else, so Billy had to sit all alone. Later, at school, the teacher began to talk about the Purple Wombat. When Billy asked her about the Purple Wombat, she was extremely angry and sent him to the principal's office. Billy went to the principal's office. The principal invited him in, and asked him why he was there. Billy said "My teacher sent me here because I don't know what the Purple Wombat is."
"Young man, go home right know! You are suspended for three days!"
So Bill went home, and his mother asked him "Why are you home so early?"
"I got suspended for three days for not knowing what the Purple Wombat is."
"You don't know what the Purple Wombat is? You go to your room this instant. Just wait until your father gets home!"
When Billy's father came home and asked Billy how his day was, Billy told him, and his father said:
"That's it! You get no dinner tonight and no allowance for a month!"
Billy, understandably upset, went to bed. At the stroke of midnight, he heard a voice calling his name. "Who are you?" Billy asked.
"I am the Purple Wombat, Billy. Follow me, Billy." said the voice.
Billy quietly snuck downstairs, got his coat, and went out into the back yard, where the voice was coming from. "Follow me, Billy." said the voice, this time from the woods behind Billy's house. Billy went into the woods, following the voice as it twisted through the trees. Finally, Billy reached a big lake, with a line of boats leading to an island in the middle of the lake.
"Follow me to the island, Billy." said the voice.
Billy stepped into the first boat, but when he tried to step into the second boat, it tipped over. Billy could not swim, and so he drowned.