Shake flashlights convert shaking energy into electrical energy by means of a high strength magnet passing back and forth through a coil of wire. The electrical energy generated is stored in a capacitor. The energy in the capacitor is then used to power a high brightness Light Emitting Diode (LED).
I have purchased a few of these over the years for myself and as gifts. They are VERY tough and worth the money. There are lots of knockoffs so be careful where you purchase it. Here's a link to the company where you can purchase them directly.
http://www.appliedinnotech.com/products/shake-flashlights/shake-flashlights.php
A shake light is not as bright as a battery powered flashlight but are great as a backup light source. Since there are no batteries to wear out, putting one in a car or boat is great insurance. They are somewhat large so don't be surpised when it shows up in the mail. They do make a smaller version but I wouldn't suggest it unless you are really tight on space, the light thrown out by the smaller version isn't worth the smaller size.
I rate it 4 'wrenches' out of 5.
