So she took some of its fruit and ate it; and she also gave some to her husband, who was with her, and he ate it.
I once heard it pointed out that the three things Eve saw correspond to the three transcendentals of goodness, beauty, and truth, respectively.
They also correspond to three powers of the soul: to sensuality, will, and intellect, respectively. By sensuality, or sensitive appetite, we tend toward things we sense as good for us ("good for food"); by will, or rational appetite, we choose goods such as the apprehension of beauty ("pleasing to the eyes"); by intellect, we comprehend truth ("gaining wisdom").
As a temptation, the fruit of the Tree of Knowledge of Good and Evil had its bases covered.
If we look a little closer, though, we can see what fools Adam and Eve were -- and, by extension, what a rotter that serpent was. Genesis 2:9 says:
Out of the ground the LORD God made various trees grow that were
delightful to look at and
good for food,
with the tree of life in the middle of the garden and the tree of the knowledge of good and bad.
The forbidden fruit, then, offers nothing to the sensitive and rational appetites that all the permitted fruits don't already have. And, as we all know,
The LORD God gave man this order: "You are free to eat from any of the trees of the garden except the tree of knowledge of good and bad. From that tree you shall not eat; the moment you eat from it you are surely doomed to die."
So Adam and Eve already have knowledge of good and bad! They already possess all the wisdom they need. As their postlapsarian sons and daughters, it's hard for us to see that something so simple can actually be complete. The subtlest manual of casuistry possible in Paradise could be printed on a matchbook:
WHAT'S EVIL: Eating from that tree.
WHAT'S GOOD: Everything else.
Adam and Eve knew what was good and what was evil before they fell. Claims that they didn't -- that eating the forbidden fruit represents their acquisition of moral awareness or free will, that the serpent in some manner brought them rationality -- are simply mistaken. The serpent tempts them into disobedience so that they can acquire... nothing they don't already have. Which they proceed to lose.
Yet it is written that, after eating the fruit, "the eyes of both of them were opened;" for that matter, God Himself calls the tree "the tree of knowledge of good and evil" (I'll leave "good and bad" for when I'm quoting the NAB). Doesn't this mean that Adam and Eve didn't have knowledge of good and evil beforehand?
Let me propose this explanation: Through their disobedience, Adam and Eve changed the rules. In the state of innocence, the only evil available to them was disobedience. Having once disobeyed, though, great sweeping vistas of evil were opened and available to them, together with the goods opposed to them. Cruelty, for example, was impossible to Adam before the Fall -- and so was clemency as such, since there was no context in which he could act in a way that moderated the punishment of another.
Thus too, "they realized that they were naked." Nakedness connotes weakness, as Fr. Alobaidi pointed out, and before the fall the weakness of Adam had no relevance to him. He lived within the context of God's ordering of Creation, and within that ordering there was no way for him to act with weakness.
It was only when Adam disobeyed God, when he decided to live by his own rules, that he had to look to himself as guarantor of his own life. At which point he noticed something, something that would have been useful to have noticed ahead of time but which couldn't be seen from the perspective of innocence (because the perspective of innocence looks at God, not oneself):
Adam wasn't capable of guaranteeing his own life. Hence the moment he ate from the tree he was surely doomed to die.
That would be an eye-opening realization, all right.