This is a really long joke, so bear with me...
A kid named Marty is in the school playground during recess. His friend Tommy comes up to him and says, "Hey Marty, want to know a secret?" Marty replies, "sure."
"Purple flower."
Tommy walks away with Marty feeling completely confused.
Marty goes over to a group of friends and says, "Hey guys, Tommy told me his secret."
"What's the secret?"
"Purple flower."
Suddenly his friends start beating him up.
After recess ends Marty goes into class looking like a mess. The teacher says, "Marty, what happened to you?"
"Well, I told my friends Tommy's secret, and they beat me up."
"What? Why on earth would they do that? What's the secret?"
"Purple flower."
"Get out. Go to the principal's office immediately."
Marty walks into the principal's office and sits before the desk. The principal turns to him and asks, "Why were you sent in here, Marty?"
"Well, I told my friends Tommy's secret, and they beat me up, and then I told my teacher the secret and she sent me here."
"What's the secret?"
"Purple flower."
"You're expelled from this school."
Marty takes the early bus home and walks in the front door. His father comes in at the sound, and asks, "What are you doing here? You should be at school!"
"Well, I told my friends Tommy's secret, and they beat me up, and then I told my teacher the secret and she sent me to the principal's office, and I told it to my principal and he expelled me."
"What would be so bad that they'd do that? What's Tommy's secret?"
"Purple flower."
"Get out. You're no longer welcome in this house."
Marty packs up a few items and is sent from his home. Night fall comes and a policeman stops him. "Hey kid, what are you doing out here at this hour?"
"Well, I told my friends Tommy's secret, and they beat me up, and then I told my teacher the secret and she sent me to the principal's office, and I told it to my principal and he expelled me, and then I went home and told my dad the secret, and he kicked me out of the house."
"Well, can you tell me this 'secret,' young man?"
"Purple flower."
"You're under arrest."
Marty is put in jail for the night. The next day, he is released. He is walking down the sidewalk, when he looks to the other side and sees Tommy. He thinks, "I'm going to ask Tommy what 'purple flower' means, once and for all." He begins to cross the road... BAM. He is hit by a bus.
The moral of the story: Look both ways before crossing the street.