This is not a new concept, there are several variations and write-ups on this project lab, but just in case one needed a one page quick version here it is. Many suggest having a balloon race across the room on a string, but I found there were not enough variations of design with that approach. This worked out much better with my groups. It is part of a bigger book due out in June 2017 "Zero Budget Labs" by Scorton Creek. I hope this helps, good luck.