"Er...the..." She frowned, trying to think of how best to explain it. "Well, the further underground you go, the hotter it gets. Water underground gets heated, and then travels back up to the surface quickly enough that it...stays warm. Ah, I'm pretty sure that's how it works anyway."
She gave him a rather sheepish smile and slipped a little further into the water, hiking the skirt of her nightgown up around her thighs. "Now, if you'd be so kind, could you turn away for a bit, Prince Ehvren? I don't think I've had a proper bath in- I don't know how long."