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. 
The card game Oh No! 99! is a keeper! It gives practice with mentally adding one- and two-digit numbers and with adding and subtracting 10 from two-digit numbers. The game encourages strategic thinking as students decide which cards to play and which to keep, and it’s also useful as an informal assessment. Read about how the game was used with second and fifth graders.