When developing for Magento, there are two ways to modify code provided by other modules - doing rewrites and observing events. If appropriate event is available for observing, it is preferred to use second method to alter other module's behavior. More precisely it is preferred to observe event of your interest, and when this event occurs, to trigger execution of your own code. But how are you supposed to know the list of events that are available for observing on your Magento installation? If you're interested to find out answer to this question, take the red pill, and I'll show you how deep the rabbit-hole goes.