I've just finished writing & delivering a new set of lectures; next week we're moving back into what is - for me anyway! - more familiar territory. At the same time I've been reading Therese Huston's book Teaching what you don't know. Huston's examples are drawn from the US tertiary system, and as you begin reading it quickly becomes apparent that 'teaching what you don't know' is a common occurrence for teachers in that sector. (And I really do mean, 'what you don't know': a science lecturer teaching a general writing course, for example. At least I'm still teaching biology!)
One of the Big Questions in a situation like this is, what & how much do you tell your students? Huston notes that in some circumstances a lecturer might not want to let on that they're teaching at the fringes of their expertise - perhaps they've previously felt that their authority or credibility have been challenged by students, & letting on now isn't going to help the classroom dynamics.
Personally, I prefer to put a positive spin on this experience. First up - I'm still teaching biology, albeit an area that's moved on a bit since I studied it. I've had to do a lot of extra reading, but I have the considerable advantage over the students in that I've already got the mental constructs into which I can scaffold my new learning. Now that I think about it, the immediacy of that learning helps me to help my students make sense of this new material as they encounter it themselves.
There's also the fact that the pace of scientific discovery is such that it's highly unlikely anyone could keep up with it. We don't have the luxury of reading papers all day, every day, so we try to read the key papers, especially those in our own areas of expertise, and know with regret that there are bound to be others that we've missed. So the odds are good that every now and then, a curious, deep-thinking student is likely to ask a question that challenges & stretches your own understanding. The big thing is not to be thrown by this.
Huston gives a number of strategies for dealing with the challenge of teaching what you don't know, drawn from interviews with a number of expert teachers who routinely teach outside their immediate field of expertise - & relish the challenge of doing so. One of these teachers, speaking of how quickly our knowledge of science moves on, comments that
"... we know different kinds of things, things that were unknown in 2005 we're certain of now. And now we know that some of what was true in 2005 was wrong. They were hypotheses and we now have the data. It's a lot of fun because students know I'm learning along with them, and we'll say "What have we learned? What are people thinking about right now? What are the big questions that are left?"
As that lecturer said, learning new things - while challenging - is also stimulating & fun. If that sense of excitement and enjoyment carries through to your actual classes, then you'll speak with passion and enthusiasm - how better to in turn enthuse your students?
What's more, the mere fact of expressing uncertainty can help students learn something of the nature of science. Don't stop at saying 'I don't know' to that curly question; take it further: 'I don't actually know the answer to that question, it's a bit outside my field of expertise. But I do know that in these circumstances so-&-so would happen, so I can hypothesize that such-&-such might happen in the circumstances you describe.' And that's a rather satisfying learning experience for everyone :-)
T.Huston (2009) Teaching what you don't know Harvard University Press