Before the darkness completely set in, the empty mountain already felt still and deserted, without signs of any humans. The fluttering, tired birds were returning home at this late hour, their wings making a rustling sound—the only noise one could hear around here. However, on a wild path, a whistling sound suddenly rang out in series.
The sounds came out piercing and clear, which were sent far out before they died away in the dense foliage. Silence then returned to the woods. Putting down his fingers, Gu Yu sighed, "I think Brother Fatty's really angry. It won't answer me."
"Don't worry, I'll send Xiao Qing to get round it," chuckled Xiaozhai.
"Get round it? Are you sure Xiao Qing is not there to pick a fight?" The idea was giving him a headache.
"Trust me, fighting also works well to ease the tension."
With that, the girl flipped her arm and the green snake, which slipped to the ground like running water, watched its master with a puzzled expression.
"You've been shut in at my place for quite some time. I'll give you one day off and come to pick you up tomorrow."
The green snake turned into a happy puppy at her words, spinning around her at her feet, all bubbly and bright. After Xiaozhai nodded in consent, the snake turned around and slithered into the bushes, reeling and swaggering.
Gu Yu had always found the communication between the two amazing. He could not help but ask, "How can it understand your words?"
"It can't, but it can grasp the meaning. The snake-controlling skill of mine requires very spiritual snakes, or they would be useless toys even if I take them in," Xiaozhai continued her slow descent down the mountain while giving Gu Yu explanations. "My master used to have a white snake for thirty years. When my master passed away, the snake was still well and healthy, but it dived into a campfire and killed itself."
"Its loyalty is worthy of respect," Gu Yu sighed. Something else then dawned on him. "Oh, by the way, where was the village you used to live in?"
"It was called Songjiang River, by Changbai Mountains."
He was mildly surprised, for he had expected it to be somewhere around Shengtian, but it turned out to be in the neighboring province. He then asked, "So, you were your master's only pupil?"
"Yes." Picking a leaf and playing with it between her fingers, Xiaozhai said, "My master was rather young back then and was living alone, accompanied by no one but the white snake. I stayed with my master for seven years before going back to Shengtian. After that, we would only meet a couple of times each year. When I got into high school, I became even busier and only managed to see my master at the deathbed. It felt wretched."
"Yea, that sounds so lonely, living without a wife and children."
"Wife and children?"
Xiaozhai turned back and glanced at him with a grin. "I never said my master was a man."
'Oh my!' Our guy was utterly embarrassed. He decided to keep his mouth shut and walk in silence from now on.
After the two exited the thorny woods and walked halfway down the mountain, they were in safe zone from there onwards. The unrecorded path they were walking on was the one Gu Yu used when he climbed the mountain with his merchandise back then, and he knew it like the back of his hand.
Therefore, they were in no hurry at all and took their time strolling down the mountain. When they finally reached the mountain foot, the sky was only slightly dimmer than when they left the valley.
The surrounding area was as deserted as always. No dwellings other than the small courtyard of the old widower stood across the road. The two of them walked down the bumpy dirt road, their backpacks on the shoulders and their arms and legs covered in scratches.
The quietness here was even more palpable than on the mountain as it had completely concealed the downtown clamor and not a hint of the bustling of the city not far away could be detect here.
The light faded as they walked on. Looking up, Gu Yu saw an inky blue sky looming overhead like a boundless dome. The bright moon hanging above was the only source of light.
He blurted out, "Hey, do you think there are immortals up there?"
"Only rocks and pits made by rocks," Xiaozhai also raised her head.
"Then what are we doing here?"
He was less certain of himself now. History had been proceeding for thousands of years and with the advanced modern technology, all those great legends were debunked by the Apollo program alone.
"We?" Without the least hesitation, Xiaozhai smiled, "We are making history, obviously!"
After getting down, they found a clinic to have their wounds treated. Luckily, the thorns were not poisonous and they were fine after applying some medicine liquid to the scratches.
Gu Yu did not book any hotel room beforehand, for he was not sure when they would finish the job. They walked into a hotel now and asked for a standard room. Neither of the two was hungry, so they went upstairs to their room right away.
He waited in the hallway while Xiaozhai entered first to change her clothes.
She changed into a white T-shirt, loose slacks and a pair of slippers. Despite her remarkable height, long, straight legs and smooth skin, Xiaozhai never wore anything that would expose her legs' shape —let alone those skin-tight leggings, she didn't even wear stockings.
Having met her a few times by now, Gu Yu had never seen her wearing anything shorter than knee-length.
Now that the two were sitting on a sofa, the threatening-yet-harmless day was finally behind them. Returning to the normal world, both were able to relax down. After a brief chitchat, Xiaozhai grabbed her backpack and dug out the files, "Take a look at these and we'll talk."
"What are these?" Taking over the files, the red mark caught his eyes first. He asked, "Are these historical times?"
"Em, I've divided them into four stages based on the historical period."
He nodded and started reading carefully by the lamp.
Judging by the historical period of the first manuscript, the material should be recorded down over 2200 years ago. Reading the first sentence, Gu Yu felt his heart racing.
"From what I have heard, immortal men who could unify themselves with heaven and earth and grasp the meaning of Yin and Yang existed in the ancient times. By regulating the circulation of essence and Qi through their breathing, controlling their minds so as to dispel all distracting thoughts, they could coordinate all muscles into a harmonious state. As a result, they could achieve longevity and live as long as the world itself, without an end. This is the way their Dao was established…"
Gu Yu raised his head involuntarily and looked towards Xiaozhai. The girl was sipping water from a white porcelain cup, acting as if Gu Yu was not even in the room.
He fought back the urge to ask her and resumed to his reading.
What Xiaozhai had been sorting out was the development of the system of Taoist practices throughout history.
When the Taoist schools were mentioned nowadays, Quanzhen and Zhengyi were the two sects coming to mind the most. Quanzhen focused on the practice of Neidan while Zhengyi on talismans. It might seem that those were the only two choices. However, that was not the case. In the period before Tang Dynasty, little was mentioned on the practice of Neidan.
Concerning Qin Dynasty, Xiaozhai had separated out the first stage of Taoist practices and named it the "ancient period", which in turn consisted of two parts, namely, the pre- and post-Spring and Autumn period.
In the pre-Spring and Autumn period, the concept of the "immortal world" did not exist and there was only the "human world". The heaven, the earth and the humans each acted according to their own natural law. The heaven and earth were created for the sake of human beings, who would adopt the Great Dao themselves.
The humans could cultivate towards four types of beings: the Perfected Men, the Supremes, the Deities and the Saints.
The Perfected Men were those who had risen above the tripartite division of the universe. They could exist for eternity and witness the creation and destruction of worlds. They were known as Heavenly Immortals to the later generations.
The Supremes could live for as long as the heaven and earth allow them. They would achieve longevity and good health. To the later ages, they were known as Earthly Immortals.
The Deities possessed remarkable physical and spiritual forms. They could not avoid death, but would not meet their end for as long as thousands of years. They were known as Immortals.
The Saints could utilize spiritual essence, with which they could nourish their body and live as long as the human body permitted. They were called Human Immortals.
Another type of special beings, who could escape death by cultivating their primordial spirits, existed as well. They could materialize in broad daylight and use other substances to form their physical bodies. They were of the same standard as the Earthly Immortals in terms of their cultivation level, but did not have the physical strength of the latter. To the later generations, they were known as Ghostly Immortals.
After the Spring and Autumn period, the concept of the immortal world became popular and the belief in following the law of the human world waned. The previous methods were partially lost and partially passed down anonymously among the common folk, and then picked up by the Taoist community. With some mix-and-match, a series of Taoist cultivating methods were created, such as the Immortal Swordsman method, the Nether Immortal method, the method of refining Qi through a focused mind, etc.
During this period, the aim of cultivation still mainly emphasized on achieving longevity of the physical body and keeping both one's body and one's mind in excellent shape.
Few records existed of this period and Xiaozhai had loaded the pages with references and her own speculations. However, she pointed out one thing in particular, which was the so-called concept of "greatness lies in simplicity". The methods the ancient cultivators adopted were extremely simple and did not contain the unnecessary bits and bobs.
The second period started from over 1700 years ago, which was named the "Ancient Immortals" period.
The Taoist methods had declined further and the chief cultivating approaches were pigu and consumption of essences. According to a certain Taoist scripture, "Grass eaters are good at walking but foolish; meat eaters are strong but ferocious; grain eaters are intelligent but have shorter lifespan; those consuming essences evade death and become Immortals." 
According to the description from another record, "This man who keeps a jujube seed in his mouth can go on without eating for as long as five to ten years. He could also remain living without breathing or moving for as long as a hundred days to half a year, appearing to be no different from a corpse."
To put it plainly, he consumed the spiritual essence between heaven and earth instead of actual food.
These methods were less powerful and contained more trivial details than they did in the ancient times; they were nonetheless still orthodox approaches. Following these methods, cultivators could live up to several hundred years with youthful appearances and seldom aged. The master cultivators could turn into immortals, elevating themselves and flying away, sometimes even bringing their pets away with them.
Similarly, it was also during this period that the Waidan practices started to crop up. 
The Waidan practice was the alchemy technique. The scripture "Divine Pills of the Great Pure Golden Liquid" contained three volumes, which emphasized that only through consuming the golden pills could one mimic the law of the heaven and achieve longevity.
Such argument was in stark contrast to the cultivation system of the law of the human world.
What was stranger, though, was that in late Han Dynasty, a fellow called Wei Boyang wrote a book named "The Kinship of the Three, in Accordance with the Book of Changes", which systematically expounded on the theories of Neidan and Waidan.
Furthermore, he had also denounced any Taoist method other than the Neidan practice as heretical, declaring them "revolting and missing the pivotal point", which deemed them heterodox tricks that would never lead to longevity.
1] ED/N: Have you ever wondered why it's sometimes Dao and sometimes Tao, everyone? Actually, they're the same thing. There are two versions because it's not quite "D" the Chinese pronounce saying it, or should I say, it's "D" pronounced in a way the English would pronounce "T", by which I mean that it's aspirated and quieter (not quite sure if it's voiceless). And, despite the pronunciation, pinyin says D and not T. So, there's a war going on among the academics, one side saying it's Dao, another saying it's Tao. In the end, it's just personal preference, and while Daoist and Taoist are both used just as often, Tao is rarer than Dao so I thought it'd look more natural, at least to the "senior" readers.
 ED/N: Some may already know it, but pigu refers to fasting. In most novels, once cultivators level up enough, they can survive for weeks or months without eating. The author hints at something else, that is fasting in order to cultivate (there sometimes appears the motif of immortals or cultivators being unable to eat earthly food lest it contaminates their bodies with impurities, it's likely that or similar).
 ED/N: As opposed to neidan (internal practice), waidan refers to external practice. If I got it right, then neidan is simply cultivating like in most novels, and waidan is cultivating by consuming alchemical pills only, without absorbing any other energy. There may be more differences, though, plus the author may later reveal her own view on that instead of whatever I can find on wikipedia, so this concludes my explanation.