Why It HappensGlass expert Mark Meshulam explains that most modern ovens use tempered soda-lime glass, which is less heat-resistant than older borosilicate glass once used in Pyrex. As ovens heat and cool, the weaker glass is more prone to stress fractures. Another culprit? Nickel sulfide inclusions — microscopic flaws invisible to the eye. Over time, these tiny imperfections can expand...