I finally got asked.

"What do you do when you don't know something?"

This was from a very nice mother of a teammate on Dale's soccer team. She had asked what grade the other kids were in and, after I'd tried to explain, I told her we're homeschooling. There was a game on, after all, and our sons were on the field. Then she asked her question.
I turned away from the game to look her right in the eye and spoke very clearly. "She's in second grade."
I don't know if she realized how insulting the question is and she certainly wasn't trying to be. She explained that her fourth-grader brings home some math she doesn't know (probably Fuzzy), so...
I tactfully pointed out that I have the teacher's manuals, so really it's not an issue. Seriously, folks, do you really think an elementary school teacher, especially in the upper-grades, has all of the answers right at the tip of the tongue? That's why they HAVE teacher's manuals.
A difference is that I've been along for the ride. I'm not going to be surprised by something in seventh grade; I'll have been along for sixth. That difference is more important than one realizes especially when it comes to math or science.
Back when I was teaching other people's kids, besides, I never claimed to know everything. My students would say, "You're a teacher, you're supposed to know everything." I don't know where they got that idea, but I'd tell them, "No, a good teacher doesn't necessarily know all the answers. She knows where to find them."

So what do I do when I don't know? The same thing employed teachers do--look it up.