“Thermal shock-resistant” means the glass is stronger and can withstand extreme temperature-changes (from boiling to freezing) without breaking. Our bottles are made with pharmaceutical-grade borosilicate glass, which is more expensive, purer, and higher-quality than most glass baby bottles, which are made with soda lime. For mothers who pump and store breast milk, our bottles can go from freezer to refrigerator to hot water, all in one step.