I had an opportunity to meet Dennis Meadows in October at the North American Simulation and Gaming Association's annual meeting. Dennis did a great session on demonstrating systems thinking principles using very short activities. He described one called Helium Stick I am planning to use it later this week. Since I found this nice description of it in a great collection by James Neill I thought I would pass it along.
update: It's not as easy as it looks. The weight of the stick needs to be less than or equal to the combined upward pressure of the team otherwise it is easy to lower the stick. I found this out while field-testing with some friends at work. Fortunately I ran into Ron Roberts at the ISPI meeting in Washington DC. I demonstrated the activity with three flipchart markers stuck together end to end. We had a great time trying to lower the stick. This lead me to making a "stick" by rolling a sheet of flipchart paper into a tight tube(newspaper would work fine too). Presto, a disposible stick. Thanks to Ron for the insight.