If you give electrical power to a motor, a part of it spins. If you spin that part manually, it actually generates power. Don't ask my how. I started thinking of something that I could use electricity for. I thought of my iPod Touch.
I then needed to come up with a way to spin the motor. I ride my bike every day to and from school, and it has a gear system to move. That's pretty awesome.
So I put plastic onto one of the metal bars and duct taped the motor to it. Then I duct taped a metal rod to the motor and the center of the gears. When I rode the bike, it actually spun the motor. Somehow, I managed to make something that didn't fail horribly the first time I tried it!
Because an iPod charges using USB, it uses 5 volts (A wall outlet is 120 Volts). That's one of the reasons you need an adapter to plug it into a wall. anything over about 7 volts would fry the battery and render the iPod/iPhone/iPad useless. Anything below 5 would just take longer to charge the battery. I was glad and mad at the same time to see that the motor only generates about 2.4 volts. This means that it's actually a reasonable way to charge it, but I also need to ride my bike for about 5 hours to charge it. I ride my bike for about 15 minutes twice a day. Another problem would be that some idiot would go and kick it off my bike or take it or something like that. Oh well, it was fun to make.