Hard on soft and soft on hard.
The teeth are very hard and as a result break rather than deform. This is something you need to know and work with when you are working with material harder than your teeth. If you are in a neck hold and bite soft muscles to free yourself it is not a big problem althouth you can still break some teeth from the thrashing but if you are trying to open a door holding the key in your mouth it can help a lot to the power transfer and your health to first take a piece of shirt in your mouth and than pick up the key. Remember to avoid harming yourself and use all that you have. Another example from the opposite side is to relax the hips and go to the ground to avoid getting stabbed by a knife or poker.