It wasn't long before someone keyed in on an adorably squiqqly line snaking up the length of a tree. A frost crack! Long ago I learned that these cracks burst open with a noise like a rifle shot as a sunny day plunges into a frigid night. I couldn't remember, though: was it the contraction of cooling wood, or the expansion of ice that caused the trunk to split? Both make sense. I pondered this as we hiked along, and also tried to spot more cracks throughout the forest.