Yesterday, Twitter changed its organizing question from "What Are You Doing?" to "What's Happening?" But the new call to action might be better suited to Hot Potato, a startup launching right now at our Realtime CrunchUp. Hot Potato is releasing an iPhone app which lets you create a stream of conversations around events based both on your location and what your friends are doing. Hot Potato is a micro-messaging app that organizes the conversation stream by events. For Hot Potato, an event can be anything that is happening right now: a basketball game, concert, party, street fair, buying a new car, or even just two friends on a bike ride. An event is whatever is happening that people want to share.