I designed the activity for students that may have not done a lot of investigation before. So it starts with a lot of modeling, and then letting them try. Students did an amazing job.
Even before doing the mental math (making the point that even number operations can be visualized, plus setting the context for the follow up activity), I asked the students to take a look at the blocks to see what they noticed. Mr. Boeve had had the students play with the blocks the day before, which is an excellent idea. They noticed corresponding dimensions, colors, different designs. These were the tiles from Algebra Lab Gear, so there's 1/2 x blocks, 1/4 x blocks, 5 sticks and 25 sticks.
I raised the problem of negatives - how could we show negatives, because algebra has a lot of those. They thought we could have two color blocks, or use some kind of design. How could we do it with the blocks we have? Maybe we could separate the positives and negatives. Nice thinking!
Sometimes I think of mini-lessons like these as equipping the students for problems. The problem list offered practice, light extension and serious problems. After 15 minutes to do their choice, which they loved having, we came back together and I asked if there were any they wanted to see me do? They suggested problems, and if there was a student to explain them, they gave it a try. One of the themes throughout was "give it a go." I made sure to ask some students who had incomplete or incorrect thinking so we could talk about that, too. Because the whole situation was different, it helped with making that safe.
In discussing the subtraction problems, they got to three separate ideas: taking away, zero pairs, and adding the opposite. The students exploring these ideas were able to give reasons for why it made sense with the blocks.
We just worked with the document camera, but if you want virtual algebra tiles, there's the National Virtual Manipulative Library tiles, a two color version from the Michigan Virtual University, and NCTM's Illuminations Algebra Tiles. None of them are ideal, but all are serviceable. It's very possible to make homemade algebra tiles, and there was a good article about that in the Mathematics Teacher: "Algebra for All: Using Homemade Algebra Tiles to Develop Algebra and Prealgebra Concepts," Annette Ricks Leitze and Nancy A. Kitt, September 2000, Volume 93, Issue 6.
Thanks to Mr. Boeve and his classes for the nice opportunity! Jill Beauchamp came along for experience with the algebra blocks, and she was a great support to the kids, so thanks to her, too.
Photo credits: Eamonn @ Flickr