Genesis 3:5

For God doth know that in the day ye eat thereof, then your eyes shall be opened, and ye shall be as gods, knowing good and evil.

1. The serpent told Eve that eating the fruit of the tree of the knowledge of good and evil would open her eyes, make her like the gods who understand good and evil.
2. At the time of the garden of Eden, according to the serpent, there were gods who knew good and evil.
3. Since the name of the tree was the "tree of the knowledge of good and evil" it stands to reason that eating its fruit would give one the ability to know good and evil.