My (three year old) daughter asked me this the other day, and I didn't have a good answer for her. I'd assumed that water up there was vapour, and so low-enoug

My (three year old) daughter asked me this the other day, and I didn't have a good answer for her. I'd assumed that water up there was vapour, and so low-enough density to just float. But it sounds like actually they stay up because rising warm air keeps them up. And when the air stops rising they fall.