The basic principal here is using the water to form a parabolic lens (magnifying glass). The parabolic lens can then magnify sunlight to create heat. As the plastic stretches under the weight of the water, the water will naturally pool in the shape of a lens. The more water you add, the larger and thicker the lens will grow which also alters the location of the focal point.  Just be careful to have the plastic securely fastened, and don't add too much water.  The weight could tear the plastic and drench whatever you have underneath with water.



Cooking with the Sun...
On a sunny day, between the hours of 9AM and 3PM, with noon being the  optimal time, the parabola will create enough heat to cook or start a  fire with. Unfortunately, because the frame requires level ground so  that the water can settle perfectly in the center, it is impossible to  aim it directly at the sun to gain maximum efficiency. However, it does  work and it can be used for alternative cooking.

In Conclusion...
While the parabolic lens solar cooker did turn out to be a great science project and draw a large crowd of curious onlookers, I deemed it to not be very practical for cooking. The frame is just too big to move around and store, and with the inability to aim it directly at the sun, it just isn't very efficient. The upside is the design is very simple, easy to build within a couple of hours, and very cheap to acquire the materials. But personally, I think I'll go with a more practical design like a Sun Oven.