It's a bike that turns both at the forks, and at the seat tube/rear wheel. You can do all sorts of weird stuff. They take about 15-30 minutes to get going, but once you figure it out, it's like riding a bike(pun intended, lol).

An example of a home made. They were also mass produced for a while until kids started cracking their skulls, etc.....

http://www.geocities.com/Heartland/P...38/watson.html