Now the serpent was more crafty than any other wild animal that the LORD God had made. He said to the woman, ‘Did God say, “You shall not eat from any tree in the garden”?’ The woman said to the serpent, ‘We may eat of the fruit of the trees in the garden; but God said, “You shall not eat of the fruit of the tree that is in the middle of the garden, nor shall you touch it, or you shall die.” ’ But the serpent said to the woman, ‘You will not die; for God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil.’ So when the woman saw that the tree was good for food, and that it was a delight to the eyes, and that the tree was to be desired to make one wise, she took of its fruit and ate; and she also gave some to her husband, who was with her, and he ate. (vv. 1-6)
This passage raises a set of questions that is only explained years and years later, and that is, who is this serpent? Does he know what he is doing? Did God make him this way? Although I'm inconsistent with this rule, I'll avoid trying to make speculations right now, because the text does not give this information.
The serpent opens with a simple enough question as to what exactly it was God had commanded. Some translations add "really", and make it sound like this question is made to raise doubt - but although it is possible that this effect is produced, the text does not read that way. The doubt will come from the lie that the serpent tells the woman, enticing her to desire the fruit, since the question really only makes the woman reaffirm what was God said...roughly.
One thing interesting about the woman's response is that it is not quite right. She has given herself a rule that is stricter than what God actually said, as far as we know, that only prohibits eating. A pastoral note can be made from this, in advising people against taking the commands of God beyond their scope, in any way. For instance, though Jesus is clear that lust is equivalent to adultery of the heart, it does not follow that admiring the beauty of another is forbidden. There may be a fine line between admiring and then twisting that admiration to an inordinate desire of a sexual nature, but it is also the fine line between sin and not sin. It must be clear to us that every time one thinks of someone else "woah, they have great eyes!", one is by no means being unfaithful to one's spouse. It is only when one then goes on to desire that person for oneself that it has lustful overtones. This is crucial if we are to avoid despair, and indeed, retain confidence that God's sanctifying grace really is at work.
Even so, the woman's response is roughly correct. Death is to follow from eating of the fruit, and here comes the lie: the serpent coaxes her in saying "there will be no punishment, it is no big deal. In fact, look how desirable it is! What gains it would bring if you ate! God only said that because he does not want the best for you." This mirrors temptation exactly, I think. In essence, we sin because we think that there will be no punishment, or not that big a punishment. We sin because we desire what is forbidden, and we think it will bring us something we lack and need. Finally, and here is the theological aspect to it, we sin because although we know that a good God would find it reprehensible, we become convinced that God is somehow withholding something from us, and that we would be better off disobeying. No sin is ever committed when we truly believe that we will be worse off when we sin, no sin is ever committed for something unappealing. Some have suggested that the essence of sin is mistrust of God, but though this is a useful generalization, these three aspects on which the serpent lie more fully spell out the root of all sin.