That afternoon the sky was grey like a woolly blanket, and a naughty rain drummed on the windows. Dash, Ziggy, Lily and Skye were all in the living room, sitting on the rug, playing at tossing a soft ball, trying to get it into a wicker basket.

"Basket!" shouted Dash every time, even when the ball ended up behind the sofa. "Well... almost a basket."

Lily laughed, Skye clapped her hands and Ziggy bounced like a spring. Outside the rain fell thicker and thicker, and it seemed like a stay-indoors day.

But right then, from who knows where, came a familiar, happy sound: "CHOOO CHOOO!"

In front of the window appeared Toby the Magic Train, shiny and smiling, with his smoke doing somersaults in the air. The children ran to open the door, and Toby greeted them in his sing-song voice:

"Hello, dear children, come with me,
today a funny place you'll see!
A magic land, all upside down,
where laughter is the game in town!"

"Upside down?" asked Lily, her eyes shining bright.

"Climb aboard now, you will see," sang Toby, "how much fun this trip will be!"

The four friends climbed aboard in a rush. The doors closed with a gentle whistle, and the little train set off through the clouds, rocking like a cradle.

After a trip full of laughter, Toby stopped in front of a big sparkling cave. The children got off and stood with their mouths wide open.

Inside the cave everything was the other way around. The floor was up high and the ceiling was down low. The rocks pointed upward like upside-down trees, and there were lots of little empty steps, just like in a stadium. All around grew crystals as big as apples, but they were dark, grey, sad.

"What is this place?" whispered Skye.

"It's the Booming Cave of the Upside-Down World," said a funny little voice from a corner. "Or rather: it's the Boooming Caaave of the Upside-Down Worrrld-orrrld-orld!"

The children turned, but they saw no one. The voice seemed to come from everywhere at once.

"Who said that?" asked Dash.

"Said thaaat-aaat-aat!" answered the little voice, and then burst out laughing. "Hee hee hee! It's me, the Playful Echo! I live here and I love to boom!"

"To boom?" asked Ziggy, scratching his head. "What does that mean?"

"Booming means a voice bounces off the rocks and comes back lots of times," explained Echo, so happy to be helpful. "Like when you shout in an empty room and your voice answers you back. I take your words and make them funny!"

To show them, she shouted: "POTATO!" And right after: "...tatooo-atooo-tooo!"

The children burst out laughing. And something amazing happened: one of the dark crystals lit up all at once, glowing with a warm golden light.

"Did you see that?" said Echo. "The crystals light up when someone laughs from the heart! But no one has laughed here for a long time, so they all stayed dark. The cave became dark and silent."

Dash stepped forward, hands on his hips and a champion's smile. "Then it's easy! We'll make the whole cave laugh! Let's hold the Topsy-Turvy Mini-Olympics! I'm great at sports, we'll win every game!"

"Yes!" shouted the others.

But in the Upside-Down World things didn't go quite the way Dash expected.

The first game was the high jump. Dash took a running start, sprang up in a huge leap and... instead of flying up, he flew down, landing on his bottom on a soft cushion of moss. "OWWIE!"

"Daaash flyyy-yyy-yy!" boomed Echo. "Dowwwn-owwwn-own!"

Skye and Lily laughed so hard that two crystals lit up together. The cave was a little less dark.

Then it was time for the race. Ziggy and Dash shot off like rockets, but in the Upside-Down World you ran on the ceiling, with your head down and your feet up. Dash moved his legs super fast, but he went backward, and ended up tucked inside a bush of sweet-smelling mushrooms.

"Where am I going?" laughed Dash, all covered in mushrooms that smelled like cake.

"Goes backwaaard-waaard-ward!" chirped Echo. "How funnyyy-unnyyy-nnyyy!"

Everyone laughed, and three more crystals lit up. The cave was starting to feel warm and bright, like a summer sunset.

Next came the obstacle race. There were lots of soft rocks to jump over, but in the Upside-Down World the obstacles weren't on the ground: they floated in mid-air like little clouds. Ziggy took a running start, sprang up to clear the first obstacle and got stuck between two little clouds, his legs pedaling in the empty air.

"Help, I'm hanging!" laughed Ziggy, swinging back and forth like a swing.

"Ziggy swiiing-wiiing-iing!" chirped Echo. "Like a saaausage-aaausage-uuusage!"

Skye ran to pull him down, but in the Upside-Down World pulling down made you go up, and they both ended up upside down laughing like crazy. Four crystals lit up all together, with a sound like lots of little bells. Now almost the whole cave sparkled, and it looked full of golden fireflies.

Then came the diving. Lily climbed onto a rock and dived gracefully, but instead of falling down she flew upward, like a balloon, and stayed hanging upside down, laughing. Then it was Skye's turn, who dived and did three twirls before landing soft as a feather.

Every game was a funny little scene, and each time the Playful Echo told it all in her wobbly voice, mixing up the words and making everyone burst out laughing. One crystal after another lit up, and the Booming Cave shone brighter and brighter.

But just when everything was going wonderfully, the most important moment arrived: the grand final of the basket throw. Dash was very serious. He wanted to make a real basket, for once, with no mess-ups.

He took the ball, aimed carefully, and was about to throw when Echo shouted: "Daaash throoow-rooow-ooow!"

Dash got scared by the noise, the ball slipped from his hands and rolled into a puddle of purple jelly. SPLAT!

"Ugh!" huffed Dash, all cross. "Echo, you ruined my basket! You make too much noise, I can't concentrate! Why don't you just be quiet for a bit?"

A strange silence fell over the cave. The Playful Echo didn't answer. She didn't even make one boom.

"Echo?" called Dash. But nothing.

Then something sad happened. One after another, the crystals began to go out. First one, then two, then ten, then all of them. The cave plunged back into darkness, cold and silent as before.

"What happened?" asked Skye, in a teeny tiny voice.

"The Echo went away," said Lily. "And without her laughter the crystals went out."

Dash felt a weight in his tummy. He looked at his shoes and understood he had said a bad thing. He looked around in the dark, and saw in a far corner a little grey cloud trembling. It was Echo, curled up behind a rock, sniffling.

Dash came closer on tiptoe. "Echo? Is that you?"

"Go away," murmured Echo, without booming. "I only make a mess. I'm strange. I ruin everything. Better if I stay here, in the dark, all alone."

Dash sat down next to her. His heart was beating fast.

"No, Echo," he said in a gentle voice. "I'm the one who was wrong. I was angry about the basket and I said a mean thing. But you know what? The basket doesn't matter at all. Do you know what really mattered? That everyone was laughing. And who made us laugh? You. You were the one who lit the crystals with your funny booms. Without you it's all dark here."

Echo lifted her head, shyly. "Really? But I'm different from you. I don't have legs to run, I don't have hands for the basket. I only know how to boom."

"And booming is the most wonderful thing of all!" said Dash, with a big smile. "No one can do it like you. Come back with us, please. In fact, I have an idea!"

Dash stood up and shouted, so loud that his voice traveled all through the dark cave: "Ladies and gentlemen, welcome to the Topsy-Turvy Mini-Olympics! The official commentator, the one who tells about all the games, the best in the whole world: the PLAYFUL ECHO!"

There was a moment of silence. Then, from the dark corner, came a little giggle. "Hee hee... commentatooor-tatooor-tooor?"

"Officiaaal-ciaaal-aaal!" answered Dash, imitating her.

And then the Playful Echo burst out laughing with joy, and came out of her corner, and filled the cave with her happy voice. "What a crazy juuump-uuump-ump! What a splendid diiive-iiive-ive!"

One after another, the crystals lit up again, but brighter than before, all together, like a thousand golden stars. The Booming Cave had never been so bright and warm. The children danced, jumped, laughed, and Echo told about everything in her funny voice, and each of her words made a new light turn on.

Dash looked at all that light and understood something important: the one who seems different, the one who does things their own way, can be exactly the friend who makes everything more beautiful.

When it was time to go back, Toby the Magic Train was waiting outside, with his happy whistle. The children waved goodbye to Echo, and she boomed: "Byyye frieeends-eeends-ends!"

Once on the train, Toby sang softly:

"A friend who's strange, or shy, or new
can be the truest friend for you.
You've seen it, Dash, this lesson's clear:
the Playful Echo's a champion dear!"

The children nodded, their eyes still full of golden light, while Toby carried them home through the clouds. And every time, in the days after, they shouted a word in an empty room and heard it come back, they smiled, and thought of their friend who boomed, down there in the brightest cave in the world.