Why is it that our eyes flip images upside down?

Basically light travels in straight lines and if light bounces off something and into your eye the light bouncing off one end and the light bouncing off the other end of the thing both have to be travelling in straight lines through your small pupil. Thus the light crosses in your lens and the image picked up by your light sensitive cells in your eye is upside down. Your brain then flips it the right way up so you don't do retarded things. The first images on the page of google images demonstrates this nicely. http://www.google.com/search?hl=en&biw=1024&bih=485&gbv=2&tbm=isch&sa=1&q=how+an+eye+sees+upside+down&btnG=Search