"Well, it's magic," said Archmage Trefortius. He scooped up a bucket of ice cubes and watched them turn instantly to water. He poured it out; the renewed cubes clattered as they rejoined their brethren. "Only the lake is affected, and only at certain unpredictable times. There's no rational scientific explanation for this; it's supernatural and unrepeatable."
Apprentice Fitz took a few experimental steps onto the surface, then wobbled hastily back. "Cold. The locals use it to chill beer. The fish thaw out fine afterward, too."
"Magic," Trefortius said again. He sighed and shook his head. "We found it at last."