Although it is true that auto glass are toughened glass, they are not unbreakable and are susceptible to scratches and cracks.
Small auto glass cracks can develop when a stone or some such similar object hits the glass while you are driving or even when the vehicle is parked under a tree. You should never neglect small cracks as they can widen with passage of time. All scratches/cracks to windshield or window glasses must be repaired immediately. If you neglect small cracks, then you may be forced at a later date to replace the entire glass which will be very expensive.
There are several round-the-clock auto glass repair shops. If the crack is small and does not interfere with the vision of the driver, then you can drive the vehicle to the nearest 24 hours repair shop. In the event, the crack is sufficiently large or obstructs the vision of the driver you may have to repair the crack on your own.
You can never predict when an emergency glass repair will be needed. If you are suddenly confronted with a windshield crack while on a vehicle journey, E8000 glue will stop the crack from spreading. This glue is available in most hardware shops. This is something like a first aid treatment.
Place the glue on the inside of the wind shield, cover with a little clear plastic strip available with outfitters. Make sure to not get this glue on your skin as it is harmful. There are also ‘Do It Yourself’ windshield crack repair kits available in the market.
If the crack is fairly large and it is not feasible to repair it, you will have no choice but to replace it. There are competent auto glass technicians who will replace the entire windshield/window glass and make the glass look absolutely original. You can protect your auto glass from scratches/cracks, by applying protective solutions to the existing or new glass.
When your auto glass develops a sudden crack, do not immediately press the panic button and order the glass replacement. Quite often than not, auto glasses can be repaired. Go to a highly reliable auto glass repairer that will repair the glass instead of straightaway replacing it.