This lab is one of my personal favorites. It is very easy to make with very inexpensive materials, and the results are very good. I think it’s one of my favorites because it is so fun to watch the students concentrate so hard to keep from hitting themselves in the head with a rubber stopper! (Inevitably, someone hits themselves in the head and we all have a good laugh. Unfortunately, many times it’s ME as I demonstrate to the students what to do!) If you can obtain glass tubes with fire polished edges (that won’t snag the nylon cord) they work best. However, if you don’t have glass tubes, PVC tubes work great. The PVC can easily be obtained at a local hardware store and cut to approximately 9-12 inches long for each lab group. Be sure to diligently monitor students to be sure that they do not exceed a hanging mass of 500 grams-many toes have been hurt due to this!