Reo and the Puzzle of Echoes

The store tunnel said everything twice.

Reo found this entertaining for six minutes.

“Basket.”

“Basket,” said the tunnel.

“Turnip.”

“Turnip.”

“Benn smells of onions.”

“Onions,” said the tunnel.

Benn stopped walking.

“I do not smell of onions.”

“Onions,” said the tunnel again.

Reo grinned.

Then the problem began.

The lower store tunnel ran beneath the cold rooms beside the root garden. Three short side passages opened from it, each ending at a wooden hatch used for passing baskets up to the rooms above.

The hatches were numbered with painted marks.

One.

Two.

Three.

A hinge had jammed somewhere.

People above were calling down instructions.

“Try hatch two!”

The tunnel returned:

“Two!”

Reo went to hatch two and lifted.

Locked.

“Not two! Three!”

The walls answered:

“Two! Three!”

Benn turned in a circle.

“Which was the real one?”

“Three.”

“Are you sure?”

“No.”

“That is not reassuring.”

They tried three.

It opened.

A basket came down.

Then a voice above called, “Back to one!”

The echo from the first side passage returned late.

“One!”

The echo from the third returned sooner.

“One!”

Reo went to the wrong hatch.

After four baskets, everyone was annoyed.

After six, two baskets had gone to the wrong cold room.

After seven, Maer Siven leaned through hatch three.

“Stop.”

Reo looked up.

Siven’s face appeared upside down in the opening.

“This is taking longer than carrying everything outside.”

“Outside has rain,” Benn said.

“We know.”

Reo listened to the tunnel.

Drip.

Scuff.

Breath.

Then a distant voice from above.

“Hatch one ready.”

“One ready,” said the tunnel.

“One,” said another wall.

“Ready,” said somewhere behind him.

The usual system was simple.

Someone above called a number.

Someone below moved to that hatch.

The tunnel turned simple into soup.

Reo put his basket down.

“What if we stop using numbers?”

Benn stared.

“The hatches are numbered.”

“That may be the problem.”

“How can the numbers be the problem?”

“They all sound like tunnel.”

“That sentence sounds like tunnel.”

Reo ignored him.

He walked to hatch one and knocked once on the wood.

Knock.

The sound returned from the side passage.

Knock.

Hatch two: two knocks.

Knock-knock.

Hatch three: three.

Knock-knock-knock.

Benn said, “Those are still numbers.”

“Yes, but they come from different places.”

They tested.

Siven called from above.

“Two!”

Reo did not move.

Instead he knocked once on hatch one.

A hand above knocked back once.

Wrong.

He knocked twice on hatch two.

Two knocks answered.

Right.

“That is slower,” Benn said.

“Again.”

They tried with three.

Then one.

Then two while someone above was talking.

The knocks were easier to locate than voices because they began at the wood itself.

But the echoes still confused them when people knocked too quickly.

Reo changed the rule.

One knock.

Wait.

If the hatch above answered the same pattern, move the basket.

If no answer came, try the next hatch.

Benn frowned.

“We are making every basket ask permission from a door.”

“Doors like attention.”

“They are hatches.”

“More reason.”

They tried again.

One knock.

Wait.

Answer.

Move.

Three knocks.

Wait.

Answer.

Move.

Two knocks.

Wait.

Nothing.

Reo looked up.

“Two is jammed?”

A voice came faintly from above.

“Two is empty!”

The tunnel returned:

“Empty!”

Benn smiled.

“That worked.”

Then it failed.

Two workers above began knocking at the same time.

The tunnel filled with wood beats.

Reo laughed.

Benn did not.

“New rule,” Reo said. “Only the hatch receiving a basket answers.”

Siven called down, “That is already how we thought it worked.”

“It did not.”

There was a pause.

Then Siven laughed.

“Fair.”

Before they trusted the new rule, Siven made them test it when the tunnel was deliberately noisy. A worker rolled an empty barrel above hatch one. Someone dropped a wooden scoop in the cold room. Benn stood halfway down the passage and coughed with theatrical enthusiasm.

Reo knocked three times.

The answer came from hatch three.

He pointed correctly.

Then Benn tried. One knock returned from the wrong wall first, but the actual answer came a breath later from the wood above him. Benn waited instead of moving on the first sound.

“That was the echo,” he said.

“How do you know?” Reo asked.

“Because I waited long enough to hear the source answer too.”

Reo nodded. That mattered more than inventing a clever pattern. The pattern only worked if people gave themselves enough time to distinguish an echo from a reply.

The next sequence ran cleanly.

Basket to one.

Basket to three.

Nothing to two.

Then a problem they had not expected.

A basket meant for one was too wide for the hatch.

Under the old system, the person above would have shouted instructions down the tunnel.

Reo looked at the basket.

Then the side passages.

He dragged it to hatch three and knocked three times.

No answer.

He knocked once at one.

One answer.

Benn said, “It still will not fit.”

Reo turned the basket on its side.

It nearly fit.

Nearly was a dangerous word for turnips.

He put it down.

“What if we stop sending this basket through a hatch?”

Benn blinked.

“That is the whole job.”

“The job is getting turnips into cold room one.”

Reo pointed to the short connecting stair at the far end of the tunnel.

It took longer.

It also had a normal doorway.

They carried the wide basket up the stair.

Siven met them at the top.

“You broke the system.”

Reo set the basket down.

“The system was already broken.”

“You broke it differently.”

“That helped.”

By late afternoon, the root stores had a new tunnel rule chalked beside the entrance:

CALLING IS FOR PEOPLE. KNOCKING IS FOR HATCHES. WAIT FOR THE ANSWER. USE THE STAIRS WHEN THE HATCH IS THE WRONG TOOL.

Benn added a final line.

REO IS NOT ALLOWED TO TEST WHETHER TURNIPS ECHO.

Reo read it.

“That rule was not agreed.”

Benn picked up the chalk.

“Turnip.”

The tunnel below answered faintly.

“Turnip.”

Reo looked at him.

Benn looked very pleased with himself.

For once, Reo decided the expected pattern was worth keeping.