Well, first you have to remember that many, many Biblical scholars believe that much of Genesis is metaphorical and/or influenced by Mesopotamian mythology.
And even if it is taken as both literal and accurate, there are a lot of interpretations that can be made. For example, many have pointed out that in Hebrew "good" and "evil" have much broader meanings. Thus, some say that the fruit from the tree actually gave knowledge of pleasure and misery. Furthermore, in this interpretation, "knowledge" might actually mean "experience". Adam and Eve, of course, knew pleasure, but after eating the tree's fruit, they also knew misery.
Of course, that's just one interpretation. Ask ten Biblical scholars about the story's meaning, and you'll get eleven different answers, and that's with two abstaining.