The case of the Knife in the Watermelon

Mr. Patch was the first grown-up to come to the Brown Detective Agency.  Hew as carrying a watermelon.
Mr Patch owned a grocer store.  He showed the watermelon to Encyclopedia.  It had a knife buried in it up to the handle.
"Find the boy who owns this knife!" roared Mr. Patch.  "Look what he did!"
Encyclopedia looked at the watermelon.  "Stabbing a watermelon isn't against the law," he pointed out.  "I mean, it's not the same as stabbing a person."
"The knife ENDED in my watermelon," Mr. Patch shouted.  "It STARTED in the window of my storeroom."
"Someone used the knife to break into your storeroom?"
"And to open my money box!" cried Mr. Patch.
"How much was stolen?" asked Encyclopedia.
"The thief didn't have time to take anything," said Mr. Patch, in a clamer voice.  "When he started to run, he tripped and fell.  His knife plunged into this watermelon.  He didn't have time to pull it out."
"Did you see his face?"
Mr. Patch shook his head.  "No, but I did see he had the letter L on the back of his jacket."
"That means he's a Lion -a member of the boys' club on Woodburn Avenue," said Encyclopedia.  "A real lead!"
The private detective stepped closer to the watermelon.  The knife had plunged into it so deeply that only the carved wood handle showed above the green skin.
Mr. Patch laid a quarter on the gasoline can.  "Find the owner of this knife, quick!"
"I'm sorry," replied Encyclopedia, thinking he would have to charge for expenses on this case.  "I'll need a little time.  I have to buy a fingerprint kit.  Then I have to dust the handle of the knife and-"
"There are n ofingerprints," said Mr. Patch heavily.  "I wiped them off."
"Y-you wiped them off?" said Encylopdeia weakly.
Mr. Patch explained.  "My cat knocked a bag of flour off a shelf.  It broke and spilled over the watermelon and knife.  I wiped off the flour-"
Encyclopedia took out his handkerchief.  He wrapped it carefully around the handle of the knife.
"That does it," he said.  "That makes it LOOK as though we have fingerprints we are trying to save.  The thief may try to wipe them off, and give himself away.  We'll have to watch all the Lions.  Let's go-"
Encyclopedia got into Mr. Patch's truck.  They drove over to Woodburn Avenue.  Four Lions - John, Frank, Corky, and Buster- were outside the club, working on the engine of an old black car.
Although few in number, the Lions were all big boys- bigger than Bugs Meany.  BUt Mr. Patch was bigger than any of them.  He had strong hands and big arms.  So the Lions listened when Encyclopedia spoke.
"Do you see this watermelon?" he asked.  "Now I take off the handkerchief.  There!  What do you see?"
"The handle..." said Buster.
"...of a knife," said Corky.
"So what?" said Frank.
"The knife," said Encyclopedia, "was used in an attempt to rob Mr. Patch's store."
"The knife..." said Buster.
"...doesn't belong..." said Corky.
"...to any..." said John.
"...of us," said Frank.
"Maybe not.  BUt the police will probably take your fingerprints," said Encyclopedia.  "If the guilty boy steps forward now, Mr. Patch will ask the police not to be too hard on him."
The Lions looked serious.  Mr. Patch looked serious.  The only boy detective in the state looked serious.
But that was all.
"It's not working the way you planned," said Mr. Patch in a whisper.  "None of them has tried to wipe the handle of the knife.  Encyclopedia nodded.  "Leave the knife in the watermelon, just as it is.  Don't touch it," he whispered back.
To the Lions he said, "The police will break up your club if they find one of you is a thief."The Lions stopped looking serious.  They looked scared.
Suddenly John said softly, "Frank owns a knife like that."
"A lot of fellows own knives with carved handles," retorted Frank.  "Cut it out!"
"You showed me yours yesterday," John shot back.  "You even tried to get me to hold it  WHy, MY fingerprintsmight be on that handle!"
"It's not the same knife," said Frank.  "So quit worrying."
"I lost my knife last month," Buster said.  "Everyone knows I did.  Where is your knife, Corky?"
"I lost mine, too," said Corky.  "This one couldn't be my knife, anyway.  Mine has a blade a half inch longer."
None of the Lions rmembered what the others' knives really looked like.  They began to argue loudly.  Each boy tried to put himself in the clear.
"Too bad," muttered Mr. Patch.  "They are scared and fighting among themselves.  But none of them has touched the knife to try to get rid of the fingerprints.  Your plan didn't work."
"Yes, it did," said Encyclopedia.  "I know whose knife it is."

HOW DID HE KNOW WHOSE KNIFE IT WAS?




SOLUTION TO THE CASE OF THE KNIFE IN THE WATERMELON

As Mr. Patch said, none of the the Lions touched the knife.  So the blade was buried in the watermelon all the time the Lions were looking at it.
In other words, none of the Lions could see how long the blae of the knife was.
But corky said his knife had a blade that was"a half inch longer" than the one in the watermelon.  That was his mistake.
He could not have known how long the blade was unless he had seen it before.
The knife belonged to Corky!