largely of the fictional element chelonium, the properties of which are apparently known to them (they do tests to look for it in Roundworld [the Discworld analogue of Earth] in The Science of Discworld), but not to readers. Magic Magic is the principal force in the Discworld and operates in a similar vein to real-world elemental forces such as gravity and electromagnetism. The Disc's "standing magical field" is essentially the local breakdown of reality, which allows a flat planet on the back of a turtle to even exist. The force called "magic" is really just a function of the relative absence of reality in the local area, much in the same way that the absence of heat is described as "coldness". Magic warps reality in much the same way as the real universe's gravity warps its space-time. The act of performing magic is, essentially, telling the universe what form it must take, in terms that the universe cannot ignore. This is very draining to magic users, due to Discworld science's Law of Conservation of Reality, which states that magic and mundane methods require the same energy expenditure. This is why most Discworld wizards store magic in a staff, which is a sort of capacitor for magical energy. On the Discworld, where magic has more in common with particle physics than Houdini, high-level background magic (most likely a reference to real-world background radiation) occurs when a very powerful spell hits, creating myriad sub-astral particles that severely distort local reality. Building a house in (or even walking into) a region where this has happened is extremely dangerous, as it is mentioned that an individual may not remain the same species, shape or level of sanity as they were when they entered. Medium levels cause odd effects, such as coins landing on their edges and turning into caterpillars. Areas with larger than normal quantities of background magic tend to display unusual qualities, even for the Disc. Very high quantities of magic can knock a hole in Discworld reality, leading to an invasion by Lovecraftian monstrosities from the Dungeon Dimensions, or, almost as bad, the world of the Elves. In the Discworld universe, magic is broken into elementary particulate fragments in much the same way that energy and other forces are in real-world quantum physics. The basic unit of Discworld magic is the thaum (from the Greek thauma, marvel), equal to the amount of