Flickerfoot and Tinseltoe, two new apprentices in the Chimney Sweep Academy's Sky-Lane division, undertake their final ascent exam up an unstable stack. Tinseltoe notices that the usual updraft is replaced by a conceptual stillness, a reading she links to the Gray Fade phenomenon affecting other parts of the system. Flickerfoot’s reliance on speed is challenged when he discovers a structural rung has been erased—not broken, but conceptually removed. Tinseltoe diagnoses this as Functional Subtraction. As they proceed using resilience twine as a temporary anchor, they see the soot collecting at the top vent, forming into spheres that suggest the creation of a Conceptual Null Object at the exhaust point, threatening a catastrophic backfill of kinetic energy.