Theories

There are many theories concerning the yukkuri. Please note that, while each theory is expected to at least make some effort at internal consistency, various theories may be completly contradicting. Each given work (be it a drawing, a comic or a text story) may pick any subset of theories to be "valid" in the context of said work.
==Yukkuri origins==
* they were always there. Perhaps they only grew more significant in numbers recently.
* Yukari did by manipulating boundaries between reasonable and unreasonable, animated and inanimated (and possibly guro and non-guro, violence and non-violence)
** she did it to give people of gensokyo a safe outlet for their violent impulses.
** she did it to turn people into psychopaths, possibly to use in another war against the moon
** she did it for the lulz.
* it was caused by Eirin's shady new drug.
* it is a Moriya shrine conspiracy.
** Templates like that are forbidden.
* it's some random magic
* the first one was a manjuu that was left for long enough to achieve sentience (this is a common motive in Japanese legends, though it usually happens with things like tools or clothes), it then reproduced and mutated to emulate people.
* the Factory was created first, and then by using SCIENCE built the prototype Yukkuri
* yukkuris are a form of Ork squig. Eatin' squigs, most likely, but evolved beyond that. An ork mob left a group of squigs in Gensokyo and forgot about it for several centuries.
*They are born from Touhou fan's secret desires to see Touhou characters doing stupid/cute stuff (And more darker desires to see violence done against them.).
*They are born from concious desires to see violence done to stupid, cute and annoying creatures.
*They came from the outside world and are starting to invade Gensokyo. They even may have overran the outside world: consider roaches and rats: both are massively successful pests. Now consider Yukkuris are larger than all but the largest rats and reproduce as fast, if not faster, than roaches. This may be also the source of their belief that they are superior to humans, as outsider humans couldn't stop their onslaught and believe Gensokyo inhabitants (humans or not) will not be able to stop them either.
*Pockets of magical energy from the residents of Gensokyo began to manifest into sentient beings.
==Physiology and locomotion==
Some people express a "Yukkuri are just magical, there's no point wondering about 'how' they do things." point of view. Despite this, some theories concerning physiology of yukkuri appeared.
*The outer layers of skin were a massive nervous system? I think nerves can perform certain movements, and in the yukkuri's case, they also double as muscles: by stretching violently the nerves in the bottom upwards, the yukkuri hops. This part is protected by extra layers of skin, to prevent the yukkuri from harming itself during the landing. Burning that part will destroy the nerve tissue, paralyzing the yukkuri.
*The dough acts as muscles. The yukkuri relaxes the bottom and contracts the rest, the contraction increases bean paste pressure and pushes out the relaxed bottom and thus propels the yukkuri. The problem with this theory is that paralyzing the bottom isn't very effective, you'd need to make it rock solid and even then the yukkuri could have some mobility... or maybe that's not a problem at all? The "burned legs" yukkuris are seen in various places, if they were completely immobile they'd just stand where the Anon left them. Also it must be noticed that the "leg muscles" just move in forward direction, one Anon did and experiment where it cut the base of a Koyukkureimu, turned it 180 degrees and pasted it back with an orange juice-based food adhesive. Then Anon put a candy in front of Koyukkureimu and started to walk to reach it, but it was getting further and further from it.
*(theory related to the Ork(Warhammer40k) origin, see above)- You know how ork weapons and vehicles only work because that's the way they're supposed to work? Tanks missing engine blocks, bolters full of shiny rocks, ect.? Well, what if these yukkuri-squigs evolved to a point where they shared this ability with orks. They can only see because they believe their eyes(which are made of beans) are supposed to grant people sight. And they jump because they think they can (but it's in fact psychokinesis/levitation).
==Crying moe==
Crying moe is a hypothetical psionic interference field that may be generated by the Yukkuri, it influences people's actions and reactions to the Yukkuri. There are various theories considering that.
* It doesn't exist. The people's act violently only due to yukkuri's behaviour, or because they want to.
* It exists. It was purposefully included in the design by some (malicious) force that wanted the yukkuri to be abused.
* It exists. It's just an unfortunate coincidence.
* It exists, and is in fact a part of defence mechanism. It works by inverting the target's normal behaviour. For example, if a wild animal could be expected to promptly eat an undefended food item, the psionic field reverses that making it leave it alone instead. Humans usually don't torture and kill sentient beings, the psionic field reverses that.
* It exists, and is in fact a part of defence mechanism. It repels predators (and other animals, because Yukkuri could as well fall prey to herbivores). Except the field either doesn't work on humans or merely annoys them. The main evidence for this theory, is the big success of wild Yukkuri- they continue to exist, despite their huge limitations, they are bigger(more food), slower(easier target) and weaker(less threat), than a "comparable" animal (they also have no claws or horns and their bite is wery weak), they should be promptly exterminated by bears and wolves. Instead the stories where yukkuri are attacked by animals are very few, and may use domesticated animals(that may react differently from the wild ones) or extreme situations.
*(Theory related to the Ork(Warhammer40k) origin, see above) the Yukkuri are heavily based on psionics, even their most basic functions originate from this. A side effect of their constant psionic emanations is that conscious mind find their presence annoying.
*It does not exist. In the case of a normal Yukkuri, the abusers do so because of their own feelings of insignificance, and the yukkuris are an excellent way to 'turn the tables'. Bad Yukkuris (Or Shitheads) are abused for their all around horrible attitude. In the wild, Yukkuris use their puffing up to intimidate wild animals, and attack ones that remain by slamming into them.
==Sentience and mental capabilities==
* Yukkuri are like very retarded humans.
* Yukkuri are like very intelligent animals.
* Yukkuri aren't stupid, but their reasoning, perception and sometimes memory are flawed- they're like humans affected by one or more cognitive disorders.
* Yukkuri are infantile, psychologically little more than manchilds: as intelligent as they can be, they still use a childish, flawed reasoning, warping severely their ability to interact with real. Imagine a really intelligent human kid and you obtain what a Yukkuri can be at its maximum potential.
* Yukkuri are like completely average humans.
===Memory===
Depictions of memory vary considerably between artists. Sometimes Yukkuri are shown to have short, little to no memory at all, while others show them to have one that is comparable to that of a human. An artist or author usually chooses which ever suits the style of work they are currently doing best, although the longer-type memory has become more popular to the more advanced storylines that can be used.
At first, it was assumed that they have little to no long time memory, and forget everything within several hours. The only exception was deeply traumathic events, that may scar the Yukkuri for life, or if the same events consistently and frequently occur. This is the way in which certain behaviours are encoded. A translated comic depicts a genetically enhanced Yukkureimu attempting to outwit Yuyuko by convincing Youmu to bring in Marisa "playmates" to harbor a select few of it's young (the others were treated by the SuperReimu as "food for Yuyuko" and dismissed them of her care except for feeding them scraps) under their hats when they had finished "playing" with Reimu. Later in the story it is depicted that a normal Yukkuri's bean paste constantly circulates in order to filter out the bad memories brought on by hardship and pain and leave only the good memories. (Thus, a yukkuri will often forget that it was bullied, tortured, or hurt even a mere hour or two beforehand. Treating them well, such as giving them sweets, in some cases make the process even faster) The enhanced Reimu's bean paste did not circulate, allowing for greater energy capacity and retention of memory, good and bad.
Another approach theorised that the more 'stale' filling is excreted from a yukkuri first, and with it the oldest memories. However, it could be heavily disputed if a yukkuri can acquire the memories of other yukkuris by eating their excrement; remember practically any material that yukkuris ingest is magically reprocessed into new filling. However, the Eat up ritual implies and confirms that Yukkuri memory is distributed in their Paste Core and their whole filling: as such, a Yukkuri eating another Yukkuri gain the full extent of its memories, and a koyukkuri, during pregnancy, gains a huge amount of the memories of its mother. It's still uncertain while stale filling doesn't carry the same properties of alive filling though.
More recently and popularly, the memory span of Yukkuris is depicted as being close to that of humans. They may still be forgetful about some things, i.e. they may seem to forget certain things from time to time, but with a reminder or triggering event, the memory often returns (although in a dangerous situation, this is often too late).
Another theory is that Yukkuris have normal memory retention, but they employ the same tactic that humans do to forget traumatic events; simply block them out. By doing this, they can maintain their mental stability, and only incredibly traumatic events force the Yukkuri's mind to rewire itself. However, some comics have depicted Yukkuris wearing other accessories and gaining some insight into the previous owner. This arises the possibility that memories are carried in the entire Yukkuri. This explains the mental instability of most hybrid Yukkuri.
===Social memory===
In most cases, the memory of social relationships fares better even if the Yukkuri have little normal memory capacity. They are usually able to keep track of their partners, children and parents.
* Their ability to tell apart nearly identically looking Yukkuris is actually quite amazing, and may be magic based (or pheromone based). Alternatively, it could be due to a process involving the accessory, as Yukkuri have been shown to be extremely fearful and hostile towards those who lack one (even if it was their own parent, or child several minutes ago).
** Several stories stress the role of the accessory in that process: by placing a different accessory on the same Yukkuri (for example a Marisa hat on a Reimu), the other Yukkuris will believe the Yukkuri has somehow been "replaced" by another one of a different kind, and act accordingly. Placing a Yukkuri accessory on a doll, or even a rock, may push the Yukkuris to believe that a new Yukkuri was spawned.
** The same process applies to humans, despite the difference in build and physical looks between different people (it has be noted that many humans have troubles in differentiating different animals of the same species, and that may apply to Yukkuris). A Yukkuri may end associating a human with its "accessory" (pigtails for a young girl or a ponytail for a young woman, the hat for a chef or a clerk in a food store, the tie on an adult male...), becoming unable to recognize him or her. For example, a family of strays was unable to remember the clerk they harassed some minutes ago while on his working shift until he put his hat back on, and a "Gold Badge Pet Reimu" was tricked into believe to live with a pleasant, loving "Mister Owner" and neighbour, hateful and bully "Mister Friend", while Owner himself just removed his tie and harassed his pet to study the effect of that peculiar behavior.
** Some older stories stretched the "magic" theory, impling the ability for Yukkuri to mystically trace one of their kind by its accessory, and homing on a human abuser, since "If you remove the accessory from a Yukkuri, you'll be stalked by all Yukkuris in the world". This ability was later lost and downtoned, possibly replaced by the "pheromone" theory, as Yukkuri are no longer able to track abusive Anons and lost accessory, but are still able to recognize different Yukkuris by their accessories, and "smell" the stench of a "dead accessory", i.e. an accessory stolen by a Yukkuri corpse.
* Some instances vary, with yukkuri forgetting even their family in minutes, unless constantly reinforced. They can easily be tricked into thinking they are someone or something else in these cases. Coupled with the lack of ability to count more than four units (see behavior), a parent Yukkuri with more than four children may end abandoning or losing a large number of its offspring without even noticing, still content of having "lots of little ones".
==Reproduction==
See main article on reproduction They have several ways of reproduction, they are all fast, pregnancy lasts hours or days at most. The child is able to speak immediately after birth, or even before.
The child is always a "clone"(at least physically) of one of the parents.
==Regeneration==
The yukkuri have strong regeneration abilities, but it's commonly accepted that a burned or cooked part will not regenerate.
If some Yukkuri, like Flandre or Remilia are known for their over-regeneration skill, it's still unknown if their regeneration skill allowed them to regenerate burned parts.
In addition to a Yukkuri's own regenerative abilities, there exists other ways for the Yukkuri to regenerate better. Orange juice has wonderful healing properties, restoring a nearly dead Yukkuri to perfect physical health. Another, less explored method, is to use the subconscious magic that the Yukkuri has. By tricking the Yukkuri into believing that healing very quickly is normal, it happens.
A possible way to do this would to make a yukkuri that is not normally very strong (I.e. Patchouli) and have it live from birth with a Patchouli/Flandre family, a lone Patchouli, exposed to the rapid healing of her Flandre sisters from small things like the occasional cut or bruise from sharp objects, would presume she would heal nigh-instantly as well and with a bit of magic and/or orange juice summoning via magic, the Patchouli would heal from anything but a devastating injury.