At #TMC15 (that's Twitter Math Camp, 2015 in California), I got to work with another math coach, a high school math teacher, and a master debater (and teacher) to create a place for discussing shared understandings of K-12 math.
The idea started in Twitter conversations and grew. I had done sessions like it at Oregon Math Leaders, so I had a vision of what it could be, but our collective thoughts made it better. Brian brought a Big Ideas article for us. It helped us see that our task was way to broad, so we focused on the big idea of equivalence (#equiv) and the specifics of the distributive property and proportional reasoning.
Watching high school teachers ooh and ahh over an elementary example was a highlight. In the end, we agreed that the conversations and process were more important than any products, although, we did talk about using similar tasks in multiple grade levels (like Andrew Stadel's cup stacking example: http://mr-stadel.blogspot.com/2015/05/the-ultimate-task-for-vertical-planning.html).
My group talked about using the Ant Man trailer in multiple grades as a hook for proportional reasoning and similarity tasks. We talked a lot about what could be done in early grades without getting too far into geometric similarity, but just using ratios. We quickly agreed that using manipulatives for students to explore these concepts was very important. If you want to see more, our thoughts are in the Google Folder on the TMC Wiki page for our morning session.
Resources on the TMC Wiki Link