Changes in temperature cause rock to expand (with heat) and contract (with cold). As this happens over and over again, the structure of the rock weakens. Over time, it crumbles. Water can get into cracks in a rock and, if it freezes, the ice will expand and push the cracks apart. When the ice melts, more water can get into the larger crack; if it freezes again it expands and can make the crack even bigger.