I use the power point to review calculating density. There are 7 problems with one bonus. The students work in groups of 3 or 4 to solve them and earn one point for their group for each correct answer. It starts off easy, but gets more difficult. It isn't a race, but if all other groups are done, give a time limit like 30seconds. Do one problem at a time and talk about the solution. They can wager points on the last problem:) The group with the most points wins. I give them a piece of candy!!! They love playing war!