Muggles' Guide to Harry Potter/Magic/Marauder's Map

Overview
The Marauder's Map is a map that shows Hogwarts and its grounds. It is one of a kind.

Extended Description
The Marauder's Map is a magical parchment showing Hogwarts castle and grounds in its entirety, including seven secret passages into the castle. These passages are unknown to most, though Filch apparently knows four. The map also shows every person's location within the castle's premises, identified by their names in minuscule writing.

When the map is inactive, it appears to be large blank parchment. It is activated by placing a wand tip on it and saying, "I solemnly swear that I am up to no good." To restore its blanked appearance, a wand taps it again as the holder says, "Mischief managed," causing the map to go blank.

When the map is activated, the parchment will reveal a writing:

Messrs. Moony, Wormtail, Padfoot and Prongs Purveyors of Aids to Magical Mischief-Makers are proud to present the Marauder's Map

The map was created by the Marauders: Remus Lupin (Moony), Peter Pettigrew (Wormtail), Sirius Black (Padfoot) and James Potter (Prongs) during their years at Hogwarts, and was confiscated by Filch because he believed it to be a dark object, though he apparently never discovered its secret. The Weasley twins stole it from his files, and it became useful in their own mischief-making; they, in turn, passed it on to Harry when it appeared that he would otherwise be unable to visit Hogsmeade. Harry has used it extensively since.

Analysis
Magic can, if handled incautiously in a story, render the plot pointless by making anything possible. If a character or an artifact is capable of doing anything, the story devolves into a simple chase of the character or artifact. Rowling makes the series exciting and dynamic in part by limiting people's abilities – not all wizards are equally magical, and even the most powerful have their blind spots – and by limiting artifacts' availability or capabilities.

The Marauder's Map is just such an artifact. It shows people's exact positions in real time everywhere in a nine-story (or more) castle, even identifying by name individuals who have never been there before. The map sees them by their psyche, thus making it proof against Invisibility Cloaks and Animagi. And when not in use, it looks like a simple blank parchment. It is necessary to show it has limitations (it is unable to reveal the Room of Requirement, for instance), and it is necessary in several situations to have it be unavailable to Harry, because it has been either borrowed or confiscated. We note that Lupin himself retains the map through much of the latter part of Harry Potter and the Prisoner of Azkaban, and Professor Moody borrows it about midway through Harry Potter and the Goblet of Fire. (This last instance, we believe Moody borrows the map because it unveils his deception, and so it is too dangerous to leave in Harry's possession.)

Questions

 * 1) How did the Weasley twins happen upon the particular spell that activates the Map? "I solemnly swear I am up to no good" does not seem terribly obvious as an incantation.
 * 2) What was there about the Map that would convince Filch that it was Dark magic? It is often described as just looking like a bit of parchment, not something Dark at all.

Greater Picture
One question that comes up is whether Professor Snape knows what the map is. He finds it in Harry's pocket in Harry Potter and the Prisoner of Azkaban, at which time it identifies its creators and insults Snape. Snape may know who Moony, Wormtail, Padfoot, and Prongs are or were, which is likely why he summons Professor Lupin to inspect it and makes such pointed comments about the map's manufacturers; Lupin is the only Marauder he can talk to immediately. Snape immediately recognizes the Marauder's Map in Harry Potter and the Goblet of Fire as being Harry's property. That, plus the Egg, causes Snape to be certain that Harry is hiding nearby under his Invisibility Cloak. Snape, by this time, knows the map's function, as he has seen it working on Lupin's desk.

Another question is how Harry gets the Map back; the fake Professor Moody borrows it, and, when he is transformed back as Barty Crouch, confesses that he used it, in Harry Potter and the Goblet of Fire. Though Professor Dumbledore expresses interest in it, we do not see it at that time, nor is Harry seen getting it back. However, he does put it to good use in later books. The author, in an interview, has stated that this was an oversight, and in the book's next edition, Harry collects it from Moody's desk while Barty is incapacitated.