Whenever you work under your car, make sure it's properly supported! This does not mean cinder blocks, blocks of wood, or anything jury-rigged. The proper method is the use a jack to lift the car, and then support it with jackstands. A jack by itself is not intended to support the car, only lift it. On the bottom of the car, just behind the front wheels (and in front of the rear wheels), there are four indentations for jackstands. This is where you should place the jackstands to support the car. Here are some illustrations on proper jack points: