This bicycle is really easy to build and to ride. It is also cheap if you recycle two old bicycles to build it. Furthermore, it is quite effective and great training for most of the muscles of your body.
Step 1: Materials
1- One bicycle (better with coaster brake).
2- Cranks (better short cranks for a child's bicycle).
3- Bottom bracket cartridge bearing, the sealed type, not the old system.
5- Two bicycle chains.
6- Some plastic tube to guide the chain.
7- Metal tube, about 25 mm inner diameter.
You may take items 2-4 from an old bicycle. I bought a long stem because the one in the bicycle was a bit too short for me. I also bought special tube for chains because it is very cheap, but you may use any plastic tube of suitable diameter.