A monowheel is different from a unicycle because the rider sits inside the wheel's circumference, rather than on top of it or outside it. Usually, a monowheel will have a circle-shaped frame with a moving track on the very outside, often made of rubber. Think of the track as a giant tire that surrounds the monowheel -- it's what's responsible for moving the vehicle forward.
How does it move? A monowheel works like a giant ball bearing. The driver and the engine anchor the inner wheel, while the engine propels the outer loop [source: Popular Mechanics]. It stays upright through the same principles as a gyroscope. As long as an external force generates motion -- in this case, an engine (although some monowheels are pedal-powered) -- the vehicle will stay in motion.
Of course, turning one of these is tricky, too. A monowheel's lack of other wheels and somewhat awkward stance makes handling difficult. A rider will typically want to keep his or her feet close to the ground so the monowheel doesn't tip over completely. In addition, if it has a powerful engine, it's unlikely that all of the power can be used in this kind of setup. Its lack of stability when forward motion is taken away makes braking especially difficult, too.
These (and a few other) attributes are what keep a monowheel from being the vehicle of the future as some people once thought it would be. You heard right -- at one time, there were predictions that vehicles with one wheel would replace cars and bikes on the road.
On the next page, we'll look at famous monowheel designs throughout the ages.