Soon the party, following an almost unrecognizable path, reached the huge Black Tower. The dim starlight could only illuminate its outline, but that was enough to make the party afraid: the tower had branches that looked like the arms of a ghost; at the top of the tower was a shape of a skeleton devil.

"Aargh..." Ranran was a little weak in the leg.

"Don't be afraid, we shall be invisible all the time. Don't worry about getting out." William said.

So the door opened automatically and then closed automatically.

Nothing inside could be seen, but this had been the best option. Because if there were lights, they would be scared to death by the blood and skeletons all over the walls. It was so quiet that even their footsteps could hardly be heard, but the quieter, the more fearful they became. They were afraid that something would come out of somewhere and attack them.

After walking about a dozen steps, they stepped on a mechanism. That was a maglev elevator. The moment it turned on, the party became extremely alert, but when they knew it was an elevator, they became less nervous.

The compass told William,

Try not to make any noise. If there's danger, I'll warn you with a bright red light, just like I did just now.

William thanked it, and it glowed red.

"That's a good compass." He thought, "I can't lose it."

The elevator stopped. They walked on, still on their guards. The two laser guns in William's hand were tightly grasped by him.

Natalie's tire hit a note. Frightened, she quickly backed away until she was in line with the others. The compass illuminated the note with the faint light of the flower.

The party read the words with horror,

My friends, this password is not difficult, as long as you can figure out.

O Z TH FI SE E FO N

"Wait a minute." Said William in a low voice, though he still felt he was too loud, "My hunch is that the letters are based on abbreviations of the English name of the numbers, so the code should be 10357849."

The note gave out light that looked like blood, and then disappeared. Almost immediately, the compass glowed bright red and showed,

Go to the elevator as fast as you can!

The party retreated crazily until they got on the elevator again. The elevator had barely started when they heard an angry voice below them,

"Damn it, they've run away!"

"What place is this... Why would someone want to catch us?" Ranran said nervously in William's ear. She held him tightly in her arms.

"Relax, Ranran. I'm sure we'll get out." Said William calmly, though he had no idea who the voice was, nor what dangers lurked in the tower.

The compass told them to get off the elevator and take extra care. They were moving at almost a snail's pace. This time everyone was watching their feet, but what was destined to happen would never be avoided.

Photon was too high and hit another note. They stood together again at once, their faces full of fear. By the light of the compass, they saw on the note a blood-stained skeleton and a question:

Write two elements in the materials in the pictures. If you don't have a clue, look at the back.

They moved carefully to the back of the note and saw pictures of hemoglobin and bones, with [Ar]3d64s2 and [Ar]4s2 below them.

These were clearly the electron configurations of iron and calcium, which William believed. But what puzzled him was that materials inside earthmen's bodies appeared on this planet. "It's really weird. There's something in this universe that shouldn't exist." He muttered inexplicably to himself, but he had to put the question aside for the moment and write down the answers.

The note also glowed like blood. After what they had experienced, they knew what to do. "Damn it, they escaped again!" The voice sounded again. But the difference this time was that they felt the sound like a sword piercing their hearts, and like a fire scorching their bodies.

"How long on earth are we going to stay here, Master?" Photon asked. William could tell that his voice trembled violently.

"I don't know, but what I do know is that the tower must have an exit." He replied.

At this floor, they came up with a seemingly perfect solution: everyone would jump up, then take advantage of the planet's feeble gravity and float around for a long time before landing. In the end, however, it just looked perfect. Unfortunate William saw the obstacle ahead, but could not slow himself in midair and could only watch as he crashed into it. As a result, the obstacle fell down and made a great noise.

The compass seemed to show with some anger,

I told you not to make a noise!

"Sorry, Compass!" Whispered William, "I didn't mean to!"

Fortunately, they were not in any danger and just had to follow the instructions.

There are five boxes with five objects inside, but each box contains the wrong object. How many boxes at least do you have to open to make sure that all the boxes and objects can be correctly matched?

"Oh, that's easy." Rebecca, who had been silent and terrified, said, "I think opening three boxes is enough, because no matter which three we choose, the remaining two must match incorrectly each other, so we don't have to check them."

Rebecca's answer was right. Again the note gave out that terrible light. The process had been etched firmly in their minds. The voice shouted more angrily than ever, "I will catch them!"

They had reached the top floor. Having learned their lessons, they decided to move more slowly. This time the question on the note was,

There is a belfry, and the clock strikes every hour and every half an hour. Every hour, the number of the clock striking is equal to the number of hour; every half-hour, the clock strikes once. A man approached the belfry one night and heard one chime, then one again; after some time he heard one chime once more, and then one again. Then he left. How long did he stay near the belfry (express it in Planck time 1E-43s)?

This made William even more confused. Why would the Planck time in this universe be the same as that in the universe where Earth was?

But the others had already begun to calculate, and the faint sound of their conversation interrupted his thoughts. "It is not logically possible to have a chime four times in a row, so there is only one possibility -- there was an interrupt." Natalie analyzed it carefully.

"You mean that man didn't hear all the chimes?" Rebecca asked.

"Yes. Three successive chimes correspond to the hour of 00:30, 01:00 and 01:30. The man heard one chime before he left. That means he couldn't have stayed until 2 o'clock. So he must have reached the belfry at 12 o'clock and caught the last chime."

Photon took her words, "In that case, he stayed for a total of 1.5 hours, which is 5400 seconds, or 5.4×1046 Planck times."

Unlike the last few times, the note disappeared, and the path in front of them was illuminated by faint spots of light all around them. The path was paved with marble stained with blood and stretched straight ahead. At the end of the path they could see the endless night and the rolling mountains outside. But they did not want to look back, because in that case they would find all the skeletons on the wall looking at them.

The compass was bright red and showed,

Danger! Don't look back! Run!

Screams rang out all around in an instant. The party was frightened out of their wits, immediately running toward the exit. Their minds were blank except for sprinting. William went first, and his ears were full of the whistling of the wind, the piercing screams, and a voice he could not understand. Ranran's face was pale, and Photon, Natalie and Rebecca were running as fast as they could.

The end of the path was nearer and nearer. But the screams behind them continued, and grew louder. Worse still, it was as if an invisible hand was pulling desperately at them.

"Hurry up! We're about to go out!" Shouted William, striding harder.

The path finally ended! In front of the party was an open window. Overpowered by fear, they had no time to think. They jumped out of the window.