Starting from Robinson Crusoe

There is strength in numbers.

Don’t underestimate the impact of turning the number of people on the island from one to three; the effect is far greater than 1+1+1.

Reasonable division of labor allows the three to maximize their areas of expertise, thereby increasing work efficiency.

Busy and bustling, by May 18, the day of the sixty-fourth mysterious arrival, Chen Zhou had completed most of the items on the schedule, with only the watchtower and beacon on the mountainside yet to be constructed.

...

Concerned about a sudden attack from the natives of the Great Tribe.

Every Saturday before going out to herd sheep, Chen Zhou would remind him to be alert and observe the northeast beach for any unusual activity. If any unfamiliar canoe appeared, he was to report back immediately.

Although Saturday wasn’t as clever as Sunday, he was very steady and reliable. Once he promised something, it rarely went wrong, making him a reassuring presence.

At least from Chen Zhou’s understanding of the two, it was more reliable to have Saturday watch the beach than Sunday.

...

Recent rewards haven’t been very exciting.

On May 18, when Chen Zhou headed to the beach, he expected to receive yet another item of little use, but instead saw a long-missed blue cube at the center of the wooden stone wall.

"Wow! A new puzzle?"

Even though the puzzles previously sent by the Space-Time Administration Bureau had tormented him a lot, true to form, without new challenges for several months, Chen Zhou sometimes found things dull.

Perhaps this was also a kind of "Stockholm Syndrome," he often thought.

Sometimes, when life goes too smoothly, various troubles arise, and revisiting the brain-burning feeling and the frustration of unsolved puzzles might not be a bad thing.

...

Having Saturday and Sunday organize the wooden stone wall——

This task now required no effort from Chen Zhou; from organizing to stacking, the two natives could complete it independently, saving Chen Zhou a lot of hassle.

Walking to the middle of the sand, Chen Zhou took down the blue box suspended in mid-air and began to scrutinize its structure.

Unlike the first puzzle safe, the second puzzle "Holy Sword," and the third puzzle "Pure Blue Hell," the solution approach for this puzzle didn’t seem to be exhaustive.

...

What Chen Zhou held in his hand was a peculiarly shaped small lock.

The surface of the lock was exceptionally smooth, with only a keyhole, seemingly simple to solve—just find a suitable key for the lock.

But the key issue lay in the material of the lock.

As far as Chen Zhou knew, modern locksmiths generally crafted keys by removing the lock cylinder and observing the shape, height, and arrangement of the lock’s teeth.

Or they removed the aluminum strip seal at the back of the lock teeth, extracting the springs and teeth, recording their spacing and height to determine the key’s tooth shape.

After creating the rough key blank, they’d insert it into the lock cylinder and adjust the tooth height little by little until they crafted a suitable key.

The common precondition for these methods was being able to take the lock apart.

However, the small cube in Chen Zhou’s hand was different from modern locks; the material was indestructible, the surface of the cube had neither screws nor seams, and disassembling it was a pipe dream.

...

"Then I’ll have to observe and guess the lock cylinder’s internal structure, trying slowly..."

Knowing he lacked the ability to destroy this material beyond human recognition, Chen Zhou muttered to himself.

He raised the lock, squinting to peer into the keyhole.

The blue material emitted a faint glow, vaguely revealing the uneven pins and wafers inside the keyhole, but it was very blurry. Creating a suitable key by this means would be extremely challenging.

"What if I use softer gold, silver, or lead to make a key blank, insert it into the keyhole, forcefully turn it, applying pressure to the key blank to leave an impression, would it make determining the tooth shape easier?"

The new puzzle was much more interesting than dull jigsaws, coupled with Chen Zhou’s eagerness to receive richer rewards.

Still standing on the sand, his mind was already active.

...

Having Saturday and Sunday diligently organize and stack the wooden stone wall, Chen Zhou hurriedly returned to the cave, putting down other work to concentrate on figuring out how to unlock this lock.

...

Chen Zhou didn’t know that his lockpicking approach was an advanced professional technique among locksmiths known as "impressioning," also called the "key impression method."

This technique sounds simple but is fraught with difficulty in practice.

The locksmith must have a deep understanding of locks and refined tactile sense.

When the key blank is inserted into the keyhole and pressure is applied towards the pins, the pins undergo subtle changes, which the locksmith needs to perceive to craft the key quickly.

Moreover, the key blank itself is relatively thin, and turning it must not apply too much force, or it may cause significant deformation, or even break in the keyhole, which would be counterproductive.

Even an experienced locksmith must use some "unorthodox methods" to assist when impressioning.

For instance, shining a light into the keyhole or illuminating the marks on the key blank to clearly see how many sets of pins, concave teeth, and protruding teeth are inside the lock.

Or using a magnifying glass for inspection.

Some even use smoke to blacken the keyhole, then insert the key blank and deduce the approximate shape of the key based on the marks left by the smoke.

And these methods were clearly unknown to Chen Zhou.

...

After returning to the cave, he went straight into the Forging Room and took out a small piece of lead plate from the materials section.

If not for this sudden puzzle, his recent plans, besides building the watchtower and beacon tower, included researching the lost-wax casting method to cast lead mini bullets, preparing for the new firearms ahead of time.

Because of this, there were many lead plates in the Forging Room, and even the graphite crucible, which was always kept on the fire wall, was moved here.

...

The hardness of lead is not high. Chen Zhou measured the size of the keyhole, drew the specific dimensions on the lead plate, picked up the steel saw, and quickly cut off a "key blank" from the lead plate.

Then he took out a whetstone to grind the uneven saw marks on the surface of the blank flat, making it smoother and more even for observing the imprints.

Once completed, the entire key blank was in a regular elongated shape.

To facilitate insertion, it was slightly thinner than the keyhole.

Realizing that this puzzle is not easily solvable, before inserting the key blank, Chen Zhou carefully examined all sides of the lead strip and sketched the few marks remaining on its surface on paper to prevent future errors that might influence judgment.

After finishing the preparations, Chen Zhou boldly inserted the key blank into the keyhole.

...

The result was very frustrating; he couldn’t even insert the key blank fully, let alone rotate it to understand the internal tooth pattern.

Seeing that only the front centimeter of the key blank entered the keyhole, Chen Zhou helplessly pulled it out, then guessed what to do next based on the few new marks on it.

...

Attempt after attempt, guess after guess, he soon found himself in the deep night.

Looking at the key blank that had been filed down like it had been gnawed by a dog, Chen Zhou placed the lock he held onto the table, feeling an overwhelming fatigue sweeping over him, nearly drowning him.

"Too difficult!

Could a person really come up with such a puzzle?

Unless I had an endoscope, how many tries would it take to find the correct answer?"

After the practical attempts, Chen Zhou finally understood that not only was matching this key complicated, but it was also tens of times harder than he imagined.

Even if he somewhat understood the general distribution of tooth height, filing the key blank was not an easy task.

...

To accomplish this task, either find an old master locksmith who has worked in the trade for many years, able to keenly perceive the physical feedback of the lock cylinder’s internal components, and reverse engineer the key’s tooth pattern based on marks and resistance to manually correct them.

Or take an unconventional path and come up with some unique creativity.

An old locksmith was naturally not easy to find, so Chen Zhou had to think of shortcuts.

Interestingly, he was recently researching the lost-wax method to cast mini bullets, and indeed came up with a daring solution—

Melt the wax and pour it into the keyhole, then attach a key handle to the base of the wax blank, finally using high-temperature resistant white clay to make a mold and encase the wax key blank, leaving a small hole to pour in molten lead.

If successful, he could obtain a lead key very close to the lock’s internal tooth pattern.

Afterward, just a minor adjustment would enable him to solve this puzzle.

...

After much consideration, Chen Zhou felt that this bold idea was highly feasible.

But before starting the attempt, he needed to conduct a series of tests on the material of the small block to avoid the scenario where the cast lead key gets stuck in the keyhole, leaving the puzzle unsolved forever.

Since the night of May 18, when he conceived the unique idea, Chen Zhou had been tirelessly testing the properties of the material to make the small block.

He discovered several characteristics of this pale blue material:

Firstly, it was indestructible, with hardness at least at the diamond level, and with excellent toughness, under Chen Zhou’s current conditions, it was impossible to damage it through violence.

Second, it was delicate and smooth. The surface of the block seemed to have been cut through space-time, so smooth that it almost felt frictionless when fingertips brushed over it.

Due to this characteristic, Chen Zhou also found that the block would not stick to any substance; even applying Fish Swim Bladder Glue wouldn’t adhere.

As for the last characteristic, it was its excellent thermal conductivity and unknown melting point.

If a thin lead strip was inserted into the keyhole and then the small block placed into a forge for high-temperature firing.

Once the thin lead strip in the keyhole melted due to the high temperature, the block would not undergo any form of deformation and would consistently maintain its pale blue color, standing out vividly amidst the flames.

These three major characteristics endowed Chen Zhou with enough confidence.

Indestructibility meant the block wouldn’t be damaged by his bold attempts.

Being smooth and non-adherent to other substances meant the poured lead key blank wouldn’t be completely locked in the passage, most likely able to be pulled out directly.

And the last point represented extremely high tolerance—

Even if the lead key blank got stuck in the passage, there was no need to worry, simply placing the block with the keyhole facing down in the crucible and then burning it with intense fire, once the temperature in the crucible reached the melting point of lead, the key blank would naturally turn into lead liquid and flow out of the keyhole.

Thus, he had infinite opportunities to retry.

...

Having developed a comprehensive plan, he rested for a night.

Mentally rehearsing repeatedly, confirming that his attempt wouldn’t result in any severe consequences.

On May 21, Chen Zhou formally began attempting his unique method for solving the puzzle.