My front teeth show very thin lines of cracking on the outsidewhich are not visible, but they are not painful. could it be a symptom of something?

Craze lines: Fine cracks in the enamel of teeth are quite common and typically are not necessary to repair. The cause can be wear and tear, force and temperature changes. Excessive force like using your teeth as tools or biting fingernails, thread or fishing line may be a cause. Enamel is a crystalline structure and can develop these fine lines that don't penetrate the deeper dentin layer or risk fracture.

Yes: Craze lines can be caused by stress placed on a tooth. This can happen over the course of a lifetime of chewing or by any other process that causes a lot of stress on our teeth.Some common causes of craze lines are biting your nails, grinding your teeth (many people do it in their sleep without realizing it), using your teeth as tools, trauma to your teeth, or simply routine use of your teeth.

Normal: All teeth have superficial cracks or craze lines. This is normal wear and tear. However if these cracks deepen or widen from trauma, they may serve as pathways for bacteria to affect the pulp, necessitating root canal.