This is gruesome. Beware. Don't watch if you're easily grossed out. The rat has a battery access hatch made out of velcro on its back, not shown in the film. If you solder a battery, make sure you do it quick - if the battery is exposed to heat from the soldering gun for too long, it will EXPLODE! You can buy feeder pets - rats, mice, and even rabbits and sometimes guinea pigs - at snake food stores. just browse the yellow pages. The original tutorial is by CANIDA on Instructables. She also did the battery access hatch and LED lights.
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