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From Discworld & Terry Pratchett Wiki
Inspiration Particles sleet through the universe, each heralding a moment of brilliance: a new symphony, a way of getting from A to B quicker than before, lines for a new play, or deeper understanding of something than was previously comprehended.
Most of them are doomed to miss, or to reveal their brilliance to a brick wall or a starling, which is totally unequipped to deal with the revelation.
Some however, hit the right mind at the right time, and a little later you are blinking foolishly in the TV lights and wondering how the hell you thought of sliced bread in the first place...
Many of these particles hit Leonard of Quirm, one of whose first inventions was a metal helmet designed to keep them out as he was accustomed to the embarrassment of waking to find he had nocturnally scribbled all over his bedsheets.
There was a famous incident in Sourcery, when Rincewind was hit by one of these particles. What came out of his mouth wasn't his and would have made sense to many a learned Wizard. However, it was said to Conina and Nijel, whereat Conina called him an idiot. Such is the lot of the inspired...