tell me why?

taken from http://www.ccmr.cornell.edu/education/ask/index.html?quid=323 "Many animals that hunt at night, including cats, have eyes that appear to glow when a light shines on them. The glow that you see is light reflected from a mirror-like structure, called the tapetum lucidum, which lines the back of the eye. Light enters the eye and passes through the retina where special cells, called rods and cones, absorb the light. Any unabsorbed light hits the tapedum lucidum and is bounced back to the retina where the light has another chance at being absorbed. Because animals with a tapetum lucidum absorb more light, they can see better in the dark than those lacking the special structure, like humans. For example, cats can see in about one sixth the light that humans require. The ability to make the most use of any available light is an important advantage for those animals which must catch their next meal at night."