Bert and Mert at the zoo.

"Let's do something special today, Bert," Mert said.

Bert was standing on his head, eating a slice of upside-down cake.

"Let's go to the zoo," Mert said.

Bert stood upright. "Good idea, Mert. I like to watch the monkeys."

They got out their bicycle built for two and pedaled over to the zoo. Bert pedaled one mile, and Mert pedaled the next mile. When they rode down a hill, nobody pedaled.

They came to a traffic light. Bert put on the brakes.

"What are we waiting for?" Mert asked.

"For the light to change," Bert said.

"What's it going to change into?" Mert wondered.

[ILLUSTRATION OMITTED]

Officer Teetlebaum waved Bert and Mert over to him, so they pulled up to the curb.

"Are you two on a case?" Officer Teetlebaum asked. (Bert and Mert were always finding lost things and solving mysteries.)

"No, we're on our bicycle," Mert said.

"We're going to the zoo," Bert said. "We're taking the day off."

"Keep an eye out for Wily Willie," Officer Teetlebaum said. "He's up to no good. That guy's always playing tricks on people."

"We'll keep four eyes out for him," Bert said.

At the zoo, Bert and Mert met the zookeeper. "What's new?" they asked her.

"The gnu's new," said the keeper, painting to a baby gnu who had just arrived at the zoo.

"New gnus are good news," Bert said.

"But we have a problem," the keeper said. "Maybe you two can help out."

Bert put on his Sherlock Holmes hat, and Mert got out his giant magnifying glass. "What's the problem?" Bert asked.

"Come, I'll show you," the keeper said.

Bert and Mert followed the keeper to where the zebra was kept. Instead of a zebra, though, there was a white horse.

"See? The zebra's gone. And this horse was left in its place."

"Hmm," said Bert.

"We'll look around for some clues," said Mert.

The keeper had other things to do, so she left them alone.

[ILLUSTRATION OMITTED]

Bert and Mert snooped around the zoo. They looked everywhere and at everything. When they came to the elephants, Mert said, "The elephants might have heard something. They're always traveling."

"How do you know?" Bert asked.

"Because they carry their trunks with them."

Soon Bert and Mert came upon a schoolteacher surrounded by students.

The teacher sighed and said to Bert and Mert, "I had to bring them on a field trip because somebody took all of the chalk from my classroom. Can you believe that?" Bert and Mert said they could.

Then Bert stumbled over a bucket that was tipped on its side. Mert helped Bert to his feet. "Are you all right?"

"I had a nice trip," Bert said.

"Look, Bert!" Mert said. "See what's in the bucket!"

"It looks like ground-up chalk," Bert said.

"And have a look at these," Mert said, taking out his magnifying glass. Someone had stepped in chalk and left footprints on the ground.

"Let's follow them," Bert said. "Do you think we'll find the zebra?"

"Who knows?" Mert said.

Then they came upon a pair of shoes sticking out under a fence. Bert cried, "We've found the thief!"

Together they opened the fence gate. There stood Wily Willie!

"Just as we thought," Mert said. "Wily Willie, you're the one who did this!"

The zookeeper came running over, and with her was Officer Teetlebaum.

"What's going on here?" asked Officer Teetlebaum. "Why, Wily Willie! I should have known you would be mixed up in all of this."

[ILLUSTRATION OMITTED]

"Where's the missing zebra?" the keeper demanded. "Does this man know?"

"He should," Bert said. "He was the one who did it."

"Did what?" Officer Teetlebaum asked.

"Grab a bucket of water and follow us," Mert said.

They all went back to where the zebra was kept.

"Now?" Bert asked.

Mert nodded.

Bert poured water on the horse, and most of the white came off. Now the horse was striped with black.

"There's your zebra," Mert said.

"Wily Willie turned the zebra white with chalk. Chalk he stole from the teacher," Bert said.

"Come along, Wily Willie," Officer Teetlebaum said as he led Wily Willie away.

Bert and Mert had solved the case. The zookeeper gave them all the popcorn they could eat.

Then they hopped on their bicycle. Mert pedaled one mile, and Bert pedaled the next mile. When they rode down a hill, nobody pedaled. Soon they were home.