Waribashi (割り箸） are those wooden chopsticks that you need to break apart. Some people rub them together as a matter of course, but this is only even needed if the chopsticks are so cheap that they are splintery. Doing this with good quality waribashi indicates that you think they are cheap, and therefore is an insult.
Tooth sensitivity is caused when the outside of teeth, or the hard enamel, wears down or gums recede and the tiny tubes in the tooth dentin become exposed. Each of those tiny tubes contains a lot of nerve endings, and when they’re left open, it can hurt when you put hot or cold things in your mouth.