The process is called freeze-thaw. 1st, liquid water gets into small holes or cracks in the rock (such as by rain or sea water splashing into them). 2nd, when the temperature drops this water will freeze into solid ice, because ice occupies more room than water this state change puts pressure and tension on the surrounding rocks. 3rd, when the temperature increases again the ice will melt back into water. It is this pressure increase and release than causes the rock to weather.