There's also a lot of variation between teeth. Smooth teeth are less likely to develop caries than teeth with deep crevices, and some people have brittle teeth

There's also a lot of variation between teeth. Smooth teeth are less likely to develop caries than teeth with deep crevices, and some people have brittle teeth that break easily. It's not just how well you brush your teeth. Another factor is what you eat/drink. If you drink sweetened drinks all day, your teeth are probably going to suffer, and I doubt that brushing teeth twice a day is going to make up for it.