We are beginning to see more bottles made of borosilicate glass coming on the market, and this higher grade of material is a welcome development. But don't let anyone tell you it's less breakable. Borosilicate glass is most widely used for bakeware and test tubes because it is more resistant to thermal shock - breaking due to changes in temperature. There may be a relationship between this low coefficient of thermal expansion and the kind of breakage you'd get by dropping a glass bottle when it's full of warm milk or formula, but it isn't, to our knowledge, one that has been studied with any rigor by baby bottle companies. The only real test is to drop it, from a reasonable height, and see if it breaks. If it breaks with lukewarm water in it, it has failed the most generous test of its mettle