Calvin and Hobbes is a classic comic strip about a boy named Calvin and his stuffed tiger Hobbes, who looks like a real, if not antropomorphic tiger, to Calvin.
Any fic at all is rather confounding for the Calvin and Hobbes canon as it is a series of short strips, page spreads, and episodic short story arcs. In fact, Calvin and Hobbes ran as a newspaper comic strip in some areas. One of its overarching themes is the fun that kids have that adults always seem to miss. Calvin groaning at icky tuna casserole, staying up past his bedtime, making ridiculous winter snowmen, being pounced by Hobbes every time he gets home from school, and declaring girls are icky are primary subjects, all of which showcase childish reactions paired with a strangely mature style of reasoning.
In other words, there is no Serious Business—unless deep, introspective monologues about the nature of things count, bearing in mind that these are usually undercut by sledding off the edge of a cliff followed by Calvin declaring that he has learned nothing. This, combined with the episodic nature of the comics, makes it very incompatible with the token fanfiction community. From the adaptation between mediums to the no-drama (beyond "Mom is making me take a BATH! OH NO!") story ideas, it's just very hard to write goodfic about.
Hence, where there is fic, a lot of it is bad. Actually, fanart is more popular.
It is not unheard of for a tiger Mary Sue to steal Hobbes' heart.
Introducing any kind of seriousness or inappropriate subject matter probably indicates badfic. For some reason, fangirls like to age up the characters and push them into romance. Depending on how bad it is, adding mature themes, gushy romance, or Serious Business may be a charge simply for how badly it clashes with the spirit of the original comic. In a short, daily comic, the tone and themes are integral to its canon.
That being said, there are quite a few good fics going around on the Pit. One of the most popular ones is called "Calvin and Hobbes: The Series" and is written by Swing 123 and GarfieldOdie. The crossovers are quite good, too.
Cardboard Box Technology
Calvin's cardboard box can serve as a Duplicator, Transmogrifier, or Time Machine. The orientation of the box indicates which is which: open side up means a Time Machine, open side to the side means a Duplicator, and open side down means a Transmogrifier. Although how cardboard box technology could possibly work may seem bewildering and probably complex or impossible, it really is not complicated at all: it works because you want it to.
A Duplicator is a device that can create an exact duplicate of anything put into it. It works best with organic or semi-organic matter. Duplicators are known to make a "boink" sound when they are operated.
A Transmogrifier is a similar cardboard box device that is capable or turning any object into any other object. Like the Duplicator, the Transmogrifier works best with organic or semi-organic matter.
A cardboard Time Machine can be made using the same cardboard box technology. Anything within the box is transported through time—and it is advisable to keep arms and legs inside the box at all times during operation. Although a Duplicator and Transmogrifier can be converted into a Time Machine, there are no reports of this happening in PPC HQ thus far.
The Department of Sufficiently Advanced Technology is known to possess at least one artifact of cardboard box technology, which Makes-Things used to mass-replicate TARDISes for agents during the Macrovirus Epidemic prior to his death. He was able to use it as a Transmogrifier to turn a macrovirus into a cockroach.