You can eat nuts but only if they are no thicker than a half a peanut or half an almond. Any thicker than that and you will need to break them into smaller and thinner pieces.
Be careful with certain bread crusts. Some are hard enough to break your appliances.
If you eat fruits with pits, be careful not to bite down on the pit; it’s much safer to simply remove the pit before eating—same goes for olives.
If you want to eat an apple, cut it into quarters and remove the core.
There are other things we sometimes put in our mouth, namely pencils, pens, forks, spoons, fingernails and other hard objects: biting down on any of these can break your the appliance.