The gears at the front are called the chain wheels. Most bikes have two or three chain wheels that look like this:
Attached to the rear wheel is the freewheel, which looks like this:
The freewheel has between five and nine gears on it, depending on the bike. A freewheel spins freely in one direction and locks in the other. That allows the rider to either pedal or not pedal -- when not pedaling, the bike coasts (another feature that tricycles and penny-farthing bicycles lack).
To change the gears, a bicycle has front and rear derailleurs. Here's a shot of the rear derailleur:
The rear derailleur has two small cogs on it that both spin freely. The purpose of the arm and lower cog of the derailleur is to tension the chain. The cog and arm are connected to a spring so that the cog pulls backward at all times. As you change gears, you will notice that the angle of the arm changes to take up or let out slack:
The top cog is very close to the freewheel. When you adjust the gears with the lever on the handlebar, this cog moves to a different position on the freewheel and drags the chain with it.
The chain naturally slips from one gear to the next as you turn the pedals.
Everything about a bicycle is simple. That's what makes it such a great machine to ride -- and also a great mechanical work of art! For more information on bicycles and related topics, check out the links on the next page.