Stationary training device, sort of like a treadmill for a bike. Rear wheel sits between two rollers, and the front wheel sits on one roller, which is driven by a belt from the rear.

Takes some practice with balance at first, but most folks learn pretty quickly.

Helps your pedal stroke and balance.

Cheap rollers can be had for $99. Best rollers like Kreitlers can get up around $300-500.

http://www.kreitler.com/

And TeddyKGB, if you can untwist oreos, and eat the filling while riding on them you'll be doing well.