Tardigrades, also called water bears, space bears, moss-piglets, flec-mites, teeneylies, or weelocks, were microscopic creatures. (PROSE: The Short Briefing Sergeant's Tale) They lived in the air and in the water, capable of surviving the coldest temperatures and living almost forever. (PROSE: Normality) Each was half a millimetre long, had eight podgy legs, and fed on lichen and moss.
A tardigrade was made of only 40,000 cells, which helped it survive in environments ranging from within one degree of the limits of normal entropy matter, to upwards of 1.5 times the boiling point of hydrogen oxide. They were also effectively indestructible, surviving supernovae, gamma ray bursts, and large asteroid impacts. They could live without food for thirty years and still be able to reproduce in only fourteen days after rehydration. (PROSE: The Short Briefing Sergeant's Tale)
Tardigrades first appeared millions of years before the Great Houses, and during the anchoring of the thread they were embedded into the Web. (PROSE: The Short Briefing Sergeant's Tale) They lived not only on Earth (PROSE: Normality) but also on millions of other worlds, with a total population of a septillion septillion. Wherever life evolved, so would the tardigrade, as its form was directly implied in the Hausanthropic equations. (PROSE: The Short Briefing Sergeant's Tale) The Eleventh Doctor compared the space-dwelling Tonnchenform to the tardigrades. (PROSE: Normality)
The Sergeant-instructor claimed that, despite their limited size, tardigrades' brains had three lobes: one maintained bodily functions; another linked each tardigrade's mind into a great collective psyche, with a debundled processing capacity large enough to run multiple nested simulations of the entire universe; the third and final lobe performing transference calculus and solving incomprehensively complex logomantic equations to edit reality, though the final purpose of their calculations was unclear. This made the tardigrades the Enemy or, at best, an Enemy tool.
The Great Houses isolated worlds of tardigrades in timestop and built engines to replicate their brains neuron by neuron, but they could not find a way to communicate with the tardigrades. The Houses were unsure whether tardigrades were even conscious or if they even knew the forces they had set into motion, but the War strategists agreed that, when the tardigrades' thoughts reached maximum observer criticality at some unpredictable time in the future, they would take control of the fixed points and redefine history in their own image. This terrified the Conquistador, but they were uncertain if the Sergeant-instructor was telling the truth. (PROSE: The Short Briefing Sergeant's Tale)
To explain tardigrades to Clara Oswald, the Eleventh Doctor got her to stare at the TARDIS column for ten seconds, then shut her eyes, count to five, and open them again. All she saw were those "tiny little things that float in front of your eyes when you look at a bright light". She hypothesised that this was just fluid being reflected back, but the Doctor corrected her.
In 2013, Clara taught about tardigrades to her Coal Hill School English class, including Courtney Woods, Joshua Adams, Zehra Jalindra, Mackenzie Brooks, Liam Mannings, Chiana Holden, Tommy Breare, Kelly Squires and Sam Kaye-Jones. They went outdoors for this lesson. She gave her students much the same instructions as the Doctor did her, except to look at the sky. (PROSE: Normality)