I believe strongly that mistakes are learning opportunities. At least that’s what I regularly tell students. But it sometimes feels different when the mistakes are mine . . . and especially when they are pedagogical mistakes that I make while teaching. That happened to me recently when teaching a lesson to fourth graders. 
I love Twitter. On November 3, 2017, I saw this image in a Tweet posted by @MarkChubb3. The image stuck with me for several days. After talking about it over dinner with a teacher friend, and then again over lunch with another, I became curious to find out what students might think. I made arrangements to “borrow” three fifth-grade classes and made plans to teach the same lesson in each class. 
A friend and I were talking recently about how much work we put into planning lessons. Even after all these years of teaching, I have to think through lessons as carefully as possible, both about the logistics and about the mathematical thinking I want to keep in mind and support. Here’s an example.