My factoring unit came on the heels of reading Sarah Carter's Halloween Candy Tax post over at Math Equals Love AND my husband's use of Hershey Kisses in a lesson of capitalism/socialism/communism.
The beauty of the clear bags and only giving students 1/4 cup is that they could spread out all of their M&Ms and count them without ever opening the bag--no spilled M&Ms during our lesson!
I gave each person a bag. My student already sit in pairs, so each pair had to determine their M&M tax. I explained that I also like M&Ms and so I was going to tax them to get my share. The tax was different for each pair. The only tax law is that I get as many M&Ms as possible while taking the same number of each color M&M from each partner.
What I wish I'd done differently...actually take the common M&Ms from them. I was trying to be nice, but I missed out on some "concreteness" (pretty sure I made up that word) by not actually taking them.