[The Scientist (OP)]

I made a solar cooker using water as the magnifying glass. But I need to know the theory behind it. Can anyone please help me? Thank you.

[JP]

A lens works because of the property called refraction.  Let's say you have a glass lens.  Light travels slower in glass than in air, so it bends when it travels from air to glass.  Even a window bends light.  Because a lens is curved in a particular way, it bends light in just the right way to focus it or form an image. 

The reason water works as a lens is that light travels slower in water than in air.  Therefore, it bends when it hits water just like it bends when it hits glass.  If you somehow get water to take the appropriate lens-like shape, it can form images just like glass.

There's all sorts of equations telling you about where the image forms and the magnification once you have a lens.  You can find more in these two links:

http://en.wikipedia.org/wiki/Index_of_refraction

http://en.wikipedia.org/wiki/Lens_(optics)