Early stages of implementing Peter Liljedahl's Building Thinking Classrooms into my 4th-grade math class. Students are working on two tasks simultaneously: 1. Make as many combinations of coins (quarters, dimes, and nickels) that add to 1 dollar. 2. The Tax Collector Problem: There are 18 envelopes numbered 1 through 18. Each envelope contains the amount of money that is written on the envelope. (i.e. one dollar in the first envelope, two in the second, etc.) The group selects one envelope. The Tax Collector then gets all envelopes that are factors of the envelope the group selected (if they select the 15, the tax collector gets the 1, 3, and 5 envelopes.) They continue selecting remaining envelopes until they select an envelope that leaves the Tax Collector with no envelopes that are factors of the selected envelope. At that point, the Tax Collector gets all the remaining envelopes. The goal is to collect the most money possible, leaving the tax collector with the least amount possible.