How do you know that?

It is inevitable with every breaking news story: Reporters will make mistakes, often big, embarrassing ones.

In the case of the Tucson shooting, the mistake was that Rep. Gabrielle Giffords had died. That was reported by both NPR and CNN, and both news organizations said they had gone to lengths to confirm the information with multiple sources.

Of course, we all know that Giffords did not die, that she miraculously survived a grievous head wound and has been making steady progress in what will no doubt be a long recovery.

One of my non-journalist friends on Facebook asked me what I thought about this awful reporting error. I replied that it always seems to happen when a story is moving and changing fast, and when it has drawn wide media coverage.

There are a couple of reasons for it. The push of competition is an obvious one. But another has to do with simple human frailty. Witnesses get confused. People fill in gaps for themselves as they try to comprehend what has just happened. They hear things from others and lose track of where they got their information. They overclaim or embellish – not always out of bad intent, but out of the human need to make sense of events, to create a narrative even when none is readily apparent.

That’s why the most important question a reporter asks when covering a fast-moving story is this one: How do you know that?

I’ve done it myself as both a reporter in the field and as the editor reviewing a reporter’s story. And on more than one occasion, I’ve seen the person I was addressing stop cold and respond: I’m not sure.

I’ve always thought the “how’’ and “why’’ questions were the most important ones a journalist could ask, and that instead we put too much emphasis on what I consider to be the easiest questions: Who, what, when and where? The how and why questions can’t be answered with a single word. They require thought and explanation.

Journalism is richer when we ask how we know things. That’s true for all of our civic conversation. Asking how we know helps us discover the origins of our thoughts and beliefs. It helps us to provide context for those who are listening to us. It can reveal common ground where we thought none existed.

And as it does for journalists, asking how we know can keep us from assuming too much, from imputing motive, from misunderstanding or making the big mistake.