You don't. Most people aren't nearly as intimately acquainted with bipolar as we are, and you're right, they DON'T understand. So what you say is "I'm fine" and move on with your day.

By the way, welcome to PC! I think you'll find the Bipolar forum a sanctuary where understanding is always just a mouse-click away. Almost everyone who posts here either lives with bipolar or cares about someone who does, so there's plenty of support to be found. We're here whenever you need us.