When you are speaking to a large crowd, you are still talking to one person at a time, just as if you were chatting to the cashier at the food store or a fellow passenger on a plane. Whether the topic is...well...the weather, or a description of a favorite camping trip, or an answer to the airborne time-passing question, "What do you do?" you are giving an abbreviated speech, complete with a beginning, a middle, and an end.