On a deeper level we are puzzled. The sin was to eat of the fruit of the “tree of the knowledge of good and evil”. If all they did wrong was to eat a physical piece of fruit, the episode was about doing or not doing what they were told.
But there has to be something symbolic in their action, symbolic with an intellectual quality.
Maimonides, whose “Guide for the Perplexed” is in a way a highly sophisticated Biblical commentary, says that we must understand the purpose of the intellect (see part 1, chapter 2). By means of the intellect man distinguishes between the true and the false.
Originally Adam possessed this capacity in full measure. He was still in a state of innocence and good and evil were theoretical concepts studied on the level of reason and logic.
But Adam had other, non-intellectual faculties like desire. When he let desire take control of him his intellectual faculty was affected. The knowledge of good and evil was no longer a matter of pure intellect.
His Garden of Eden days were over.