First time was pretty nervewracking, and I still get anxious before teaching, although once I get into the swing of things I calm down. My first day I learned an important lesson: performance anxiety is real. I was doing some basic probability analysis that I figured I could do off the top of my head and made an absolute mess of it and had to start over! So now I make sure that regardless of how much time I have to prep I make notes that feature fully-worked examples and exactly-worded definitions. (Much of the time my notes are just that and I can do the motivation and exposition bits off the cuff.)

I was only a couple years older than most of my students when I started and I was concerned that they'd doubt my knowledge and argue about stupid stuff, but that turned out to be entirely wrong. I'd given plenty of presentations in classes before, so I was at least a little used to standing in front of a classroom, but teaching felt different because the students were really paying attention to what I was saying for a change. Definitely a sobering moment when you realize you could say just about anything and a room of 50 people would just take your word for it!

A lot of my students have been very smart, and I really love the oddball questions they come up with, even if I don't always know how to answer them.

Teaching takes a lot out of you--even though I don't feel tired while I'm in the classroom, I pretty much have to schedule a half hour or so after class to sit and drink coffee and do mindless tasks because I need a bit of time to mentally recover.