Putting boiling water on a frozen windshield will not cause the windshield to shatter. Vehicle windshields are constructed using laminated safety glass, which is a piece of polyvinyl butyral between two or more pieces of glass. However; there is a good chance if your windshield has a small chip or crack, some of the vapour or liquid may seep into the opening and when refreezing happens the liquid will expand causing the glass to separate into a crack. It is for this reason it is always best to repair any chips before and during the winter season. Chip repair only takes 20 minutes and is usually cover by insurance without a deductible. Insurance companies would rather waive the deductible than have you claim a full replacement. Call to day to schedule your repair.