Billow

Billow drifted up toward the workshop ceiling, riding the warm air off a lamp, and called down to the older club below. "Notice where I am? Up here, where it's warmest. That's not an accident — it's my whole primitive. Warmth makes things spread out. Heat needs room. Once you see it, you'll spot it everywhere — in the air, in metal, in water, in the whole sky."

She let herself sink as the air cooled. "Here's the mechanism. When you heat almost anything, its tiny particles jiggle harder — you've met that from Amass. But jiggling harder means each particle knocks into its neighbours more forcefully and needs more elbow room. So the whole substance spreads out — it takes up more space than it did cold. That's called thermal expansion. Warm air, warm metal, warm water — all of them swell, just a little, when heated, and shrink back when cooled."

A young frog named Pib looked up at her. "But why do YOU float up there, then? Spreading out isn't the same as rising."

"Ah — it leads straight to it," Billow said. "When a patch of air spreads out, it has the same particles in more space, so it's less dense — lighter for its size than the cool air around it. And lighter-for-its-size things float up through heavier ones, the way a bubble rises through water. So warm air rises, cool air sinks to take its place, gets warmed, and rises in turn — a rolling loop called convection. That's why the ceiling's warm and the floor's cold, why a hot-air balloon lifts, why the wind blows and the weather churns. It all starts with heat demanding more room."