The master clock kept town time. It was wound by a heavy lead weight that fell slowly, keeping the gears moving. Mr Calder explained that the missing weight was not important for its heft alone. Fixed to its side had been a thin brass plaque, engraved with a coded sequence of numbers – the very code that kept the master clock wound to perfection. That clock, in turn, sent out a faint electromagnetic pulse each hour, nudging other clocks in the city to stay true. Without the plaque, the winding schedule faltered, and the city’s time began to drift like ripples across a pond.

“Someone may have thought the weights were worth something as scrap metal,” Calder murmured, “or perhaps they mistook the plaque for a piece of gold. Either way, they never realised that the smallest part was the most precious, the heartbeat that kept the whole city in step.”

Amira ran her fingers along the empty hook where the weight had hung, feeling the chill of its absence. Her mind worked with quiet precision, sifting through the options until a hidden pattern began to emerge. With a sharp glance at the others, she signalled that it was time to begin.

They found scuff marks on the back door, a smear of oil on the threshold, and a series of tiny numbers scratched on the inside of a drawer: 54, 36, 18. Lucy frowned. “It’s a puzzle in numbers and movement,” she said. “Someone who knows clocks did this.”

At the heart of the puzzle were gears, swings, faces and missing things. The children divided tasks. Oliver inspected gear patterns, Emma mapped the winding rope, Jack checked weights, while Amira did the thinking that turned loose ends into leads.

The great clock loomed above them like a golden giant, ticking softly in the shadows. Behind its glass face was a maze of brass gears, each one turning, clicking, and whispering as if sharing a secret.

Gear A was the largest, its rim studded with 54 shining teeth. Every tooth caught the light like a spark as it moved. Gear A turned two full rotations every minute, steady and smooth, the heartbeat of the clock itself.

Right beside it, Gear B looked tiny in comparison. It had only 18 teeth, but it was fierce and fast. Each time Gear A turned once, its teeth pushed against Gear B’s, forcing it to spin in the opposite direction. Because Gear B was smaller, it had to work harder, spinning three times for every single turn of the big gear.

The gears glimmered like a living machine, big and small working together, slow and fast, strong and quick. The air hummed with rhythm: click, whirr, click, whirr. The sound was steady, almost musical, like a secret code of time itself.

“The small wheel turns because the big one moves it,” whispered Lucy, her eyes wide.

Jack nodded slowly. “And if even one tooth is missing,” he said, glancing up at the clock’s trembling hands, “the whole rhythm breaks.”

They exchanged a look. Maybe the answer to the mystery wasn’t hidden in the clock’s face at all, but in something that wasn’t turning the way it should.

How many full rotations does gear B make in four minutes?

❌ Focus on how many teeth pass the contact point each minute. In this situation, the saying “tooth for tooth” really does apply – quite literally!

✅ Well done!

The clock’s pendulum had once swung with the regularity of a steady heartbeat, its motion a gentle whoosh that seemed to whisper time itself.

Mr Calder explained that when the master clock was working properly, the pendulum swung 180 times in 15 minutes, each swing marking the passing time like tiny footsteps echoing through the quiet workshop. Emma and the others watched in fascination, mesmerised by the rhythm, which felt almost alive. A silver thread weaving the present into the past, keeping the city’s hours in perfect synchrony.

How many times did the pendulum swing in eight minutes?

❌ Start by working out how many swings occur in one minute.

✅ Exactly! The pendulum is like a heartbeat: steady, steady.

A smaller, beautifully decorated clock stood proudly inside a sturdy rectangular wooden case, measuring 40 cm in length and 30 cm in width. Nestled within this case was a decorative inner panel, the very stage where the clock’s ornate dial performed its silent ballet of numbers.

This panel was inset neatly from the edges, leaving a polished wooden frame all around, 5 cm wide on every side. The frame’s rich grain caught the light, glinting like tiny rivers of gold, and seemed to hug the dial as if protecting its secrets.

The children realised that to measure the wooden frame itself, they would need to think carefully about the space it covered.

What is the area of the wooden frame (the border of wood) in cm²?

❌ You only need to calculate the area of the front of the frame. Think of it as two rectangles: a large outer rectangle (the whole case) and a smaller rectangle (the dial panel) placed inside it. The wooden frame is the part of the larger rectangle that is not covered by the smaller rectangle.

✅ That’s correct! The clockmaker would hire you on the spot!

The children peered into the inside drawer of the master clock. Along the worn wood, a curious sequence of numbers was scratched: 3, 6, 11, 18, ?. It looked almost like a secret code left by someone in a hurry. Oliver traced the numbers with his finger. “These aren’t random,” he said. “Each number seems to grow from the last, but not by the same amount. Maybe it’s a pattern in how much they increase each time.”

What is the next number in the sequence?

❌ That doesn’t fit the pattern.

✅ That fits the pattern!

The master clock wound itself using a heavy lead weight, which descended slowly, like a tiny sun sinking in the sky.

When the clock was correctly wound, the weight dropped 5 millimetres every 15 minutes, its steady journey keeping the pendulum swinging and the gears ticking in perfect harmony.

To keep the clock running smoothly until the next winding, the weight needed to descend a total of 72 cm. The children realised that calculating how long it would take for the weight to reach the bottom was a puzzle in itself, a race against time, measured in centimetres and minutes, hidden within the rhythm of the falling weight.

How many hours will it take?

❌ Try dividing distance by speed (cm ÷ cm per hour).

✅ Perfect! That should give the clockmaker enough time to repair the clock.

While checking the weights, Jack noticed footprints leading to the back lane where the cobbles glinted. Beneath a puddle lay a thin strip of brass – not the weight itself, but the brass plaque with a coded set of numbers it once carried.

Oliver traced the footprints. “They head toward the market, then vanish. The thief must have panicked and dropped it.”

Emma frowned. “Or perhaps they thought it was gold, and when they realised it wasn’t, they ditched it and fled.”

Mr Calder fixed the weights back onto the master clock and pressed the brass plaque into place, tracing the engraved code with a steady hand. The pendulum shivered, then swung into motion. Slow, sure, and steady as breath returning after a long silence. Calder’s lined face softened into a smile.

“We hunt for grand causes,” he said gently, “yet it is the tiniest pieces that keep the world in rhythm.”

Emma whispered, almost to herself, “A tiny clue solved the biggest puzzle.” Oliver nodded, his eyes following the pendulum’s swing. “And without it, the whole city would have lost its heartbeat.”

Case Closed