If you're stuck in a sinking car, or outside your car and desperately need to get inside, you have plenty of ways to break that window, but one of the best might be right there in your car anyway -- a simple sparkplug will do the trick with ease. This video from National Geographic shows you how it works.
Brian Brushwood explains in this video that if you break off a chunk of ceramic from around the sparkplug and use it against the car window safety glass, it will shatter magnificently. The direct force on a small area applied by the sparkplug ceramic is enough to punch through the safety glass, which is designed to absorb great force over a large surface area without breaking (which is why a hammer, for example, doesn't work very well.)
I've seen people do this with the sparkplug ceramic, and others do it with the point of the sparkplug itself, but throwing the ceramic at the window is probably safer from the outside (since the glass is designed to shatter outwards if it breaks). We still think keeping a safety hammer and seatbelt cutter on your keychain or in your car is a better long-term idea, but this definitely works in an emergency. If you don't have either handy, you can always use your car headrest .