I love this little machine at the San Francisco Airport. It’s in a section at the end of the concourse for United flights, where there are a few science-oriented toys for kids. Every few minutes, if not disturbed, a tornado will form in the smoke. If someone walks past the machine, the breeze they create will slowly push the base of the tornado off the platform and dissipate it. So cool.
There’s another good picture of a machine like this from the Exploratorium, here on flickr.