“At least we’re not underwater now.” Zena said. “But what are we going to do about the shark?”

Pip felt sorry for it, bouncing helplessly on the ground. He looked around. It was dark, apart from a few glowing camp-fires. They were in a cave and it was very cold.

“Look,” Zena whispered. “Something’s happening to the shark!”

The shark was wriggling wildly and changing shape. Its tail shortened, its mouth grew smaller and its body shrank. Before they knew it, Pip and Zena were staring at a boy curled up on the ground. He was young, messy-haired and dressed in grubby animal furs, and glaring at them angrily.

“Why didn’t you stop?” the boy shouted.

“Excuse me?” Zena said.

“I was swimming above you, asking if you wanted to play.”

“But you were a shark!” Pip said. “We thought you wanted to eat us.”

“That’s what everyone thinks. Only a few sharks attack humans. Didn’t you know that?”

“We’re sorry,” Zena said. “You’re not a shark anymore, by the way.”

“I know!” The boy stood up. “I don’t mind being human, though. Where are we?”

Suddenly, a low grunt came from the shadows. Three big figures were walking towards them! Pip and Zena yelped. Then they heard a little mew. It was Libro, followed by three Neanderthals.

“Libro!” Zena said. “You made some new friends.”

Pip grinned. “So did we.” He turned to the boy beside him. “What’s your name, by the way?”

The boy laughed and replied. “You can call me Sharky.”

The Neanderthals waved happily.

“Me Gub,” one said, stepping forward.

“Me Mugg,” another said.

“Me Archibald,” the last one said with a bow.

Pip and Zena giggled.

“Where’s the next door?” Zena asked. “That’s how this works, isn’t it Libro? We keep chasing you from world to world?”

Libro jumped into Archibald’s arms and nuzzled against the Neanderthal’s ear. He nodded, then bent down and began writing something with his finger on the dirty floor.

“Here’s the next word,” Pip whispered.

Sure enough, another word appeared on the ground. Your.

“Tell the world your…,” Zena said. “There’s more to come. But what?”

Pip pointed to the closest campfire. “Look! It’s the next door.”

Libro leapt from Archibald’s arms and strolled towards it.

“Here we go again,” Zena said. “Sharky, are you coming with us?”

Sharky nodded. “This is much more exciting than swimming in the sea!”

Libro jumped through the door. With a deep breath, the children leapt after him.