Below is a short lesson that I recorded for the Math 155 Elementary Mathematical Models course. I used to think that you never got a second chance to give a lecture. But with today’s video technology, that’s just not true. Every teacher has bad days just like any other human being. But when you’re trying to explain an especially difficult concept on that day, it can feel like you lost something important. Luckily I was able to go back and record this video that does a much better job of explaining the idea, when I had time to think about where my students were getting lost and how to best approach the pitfalls that they were encountering.