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Narrativium
From Discworld & Pratchett Wiki
The most common element on the disc, although not included in the list of the standard five: earth, fire, air, water and surprise. It ensures that everything runs properly as a story. For example, if a boy has two older brothers, chances are they will go on a quest. The first will be strong, and fail because of his stupidity, the second will be smart, and fail because of his frailty and the youngest brother will then have no choice but to go out, succeed and bring fame and fortune to his poor family. This phenomenon is also known as narrative causality. The application of this phenomenon appears to be governed by some loosely formulated laws.
The Wizards have found that the Roundworld has no Narrativium at all, and are amazed that it can still function.

