The freezing of water crack boulders because of water expansion when it freezes. Liquid water is a jumble of water molecules. When these freeze, the molecules line up uniformly and take up more space. This outward pressure from the large amount of space needed for ice is what cracks boulders.
Work Step by Step
Water, unlike most substances, is more dense as a liquid and less dense as a solid. This is why ice floats in liquid water. Water molecules freeze in a crystalline lattice that is more spaced out than liquid water, expanding and forcing the surrounding rock to crack.