NOTE:  There was much more to be said about the first verses of Genesis 3 than could be said in the last column.  Here is another go at it, with little duplication and no contradiction.--EW

 Now the serpent was more crafty than any other beast of the field that the LORD God had made.

 He said to the woman, "Did God actually say, ‘You shall not eat of any tree in the garden'?" And 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 midst of the garden, neither shall you touch it, lest you die.'" But the serpent said to the woman, "You will not surely 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.  Genesis 3:1-6 ESV

It's time now to talk about sin.  I hope you aren't offended. Readers familiar with the Bible have been expecting the subject to come up, because without sin the Bible--if  it had even  survived into the modern era--would be a pretty tiny book!    

The serpent (or snake) introduced temptation, not sin, into our world. Temptation is not sin; sin is but one response to temptation. Resisting it and ignoring it are two others available to humans (God deals with sin  in ways unavailable to us, though  for our good if we avail ourselves of them.) Temptation may be no more than an idea, an abstraction, and a false one at that, while sin is the act of treating the abstraction as true by translating it into action. In this story, the serpent draws Eve into a conversation about God. After it grabs her interest, it contradicts God.  God had said, "If you eat the fruit of the tree in the center of the garden, you will die." But the serpent says simply, "No you won't," and suggests that God is just trying to keep her away from the really good stuff.   

At this point, a modern homemaker  confronting a snake-oil salesman at the door might remember her mother's way of handling such situations: "I'll talk it over with my husband when he gets home," or in some cases "I'll  discuss it with our pastor," or "with our  doctor," then shutting  the door. But Eve had seen the fruit, which looked really good to eat, so she didn't want to think it over. She took the way her descendants have followed ever since: So what's  the big deal? It's just a piece of fruit. She took a piece of it; then, being a loving spouse, she took one for her husband too.

 What was the big deal?  From the get-go, the Bible was written with the assumption that God knew what he was doing, why he was doing it, and what he was  talking about.  The serpent either didn't know or didn't care, but Eve pretended it did. 

The pretending was not the sin, but it enlarged the temptation and eased the transition  from abstract temptation  to concrete action  when she took and consumed  the forbidden fruit.  

I'll save you from thinking of your own parallels,  when you said after the fact, Why did I do that? That was really a dumb mistake. Here's an editorial suggestion for your memory:  if you knew better, it was not a mistake. You make a mistake when you are trying faithfully to do what you are told and it doesn't come out right. That doesn't apply to a case in which you knew what your parent or teacher said, but pretended to yourself that they meant something else. 

When God is the one who said it, it's always a big deal not to believe Him.