I like to use Matisse to teach 2nd graders about shape. The last 2 years we've spent time learning about Matisse and his artwork, then the students used construction paper to create their compositions. Some of the students did a nice job but most struggled with the idea of overlapping while maintaing some sort of balance. (Gluing so many layers that you can no longer see the cut shapes was common.) To try to remedy that we painted the background with paint and created the foreground by painting with scissors.
The first class period was spent on introduction- we watched the video of Matisse painting with scissors, above, then a PPT presentation of his artwork- and creating the background. The students used rulers (if they wanted) to divide their 9x12 inch paper into geometric shapes. The students were given tempera paint in the primary colors to fill in their spaces. In the second class period, the students used construction paper scraps to make organic shapes. When we discussed Matisse's artwork, the students liked to point out "ish" shapes since we had recently read the book by Peter H. Reynolds. I asked that one of their organic shapes be an "ish" shape and some could just be free form. The students this year actually did a pretty good job controlling their glue but some still can not understand (or choose to ignore) the "just a dot, not a lot" reminder. :)
This student missed the first day so I asked her to start by selecting some large geometric shapes to glue down before moving on to organic shapes.