They are very unlikely to break. I know at least the better ones are made at the same places as scientific glassware and are made of Pyrex, which is very durable. Like Sharniqua said, they may not survive a large fall to a hard surface, but they will stand up to the task at hand. They also have other benefits. They can be pre-warmed in warm water and will hold the tempreture for quite some time. They hold lube better than rubber or plastic. And, they are dishwasher safe so they can be sterilized between uses/ users.