The most common cause of cracked car windows are airborne rocks, but there are many other possible reasons for car windows to shatter. Damaged windshields should be repaired immediately, as there is a chance for the crack to spread.
Although rocks or debris are the most common reasons behind a cracked windshield, other factors, such as hail, could play a part. While cold temperatures do not crack glass alone, it is important to never use boiling water to defrost windows as this is sure to cause problems. Also, drivers should avoid being directly behind a truck or trailer at high speeds, as they are notorious for kicking up small pebbles or debris.