Presenter notes: Open by reminding them of the loose end from Unit 2. Three angles did not fix a triangle, because a small 60-60-60 and a huge 60-60-60 both fitted. At the time that felt like the method failing. Tell them today they will find out that it was not a failure at all, it was a discovery, and that the whole family of same shape different size figures has a name. Framing the lesson as the resolution of an earlier puzzle rather than as a new topic gets much better attention.