What Constitutes Forbidden Magic?

When R. Eliezer fell sick, R. Akiva and his companions went to visit him [ . . . ] [and R. Eliezer said,] “I have studied three hundred”—and some state three thousand—“rulings about the planting of cucumbers, and no man, except for Akiva ben Joseph, ever questioned me about it. For it once happened that he and I were walking on a road, when he said to me, ‘My master, teach me about the planting of cucumbers.’ I said one word, and the whole field filled with cucumbers. Then he said, ‘Master, you have taught me how to plant them, now teach me how to uproot them.’ I said one word, and all the cucumbers were gathered in one place.” [ . . . ]

But how did [R. Eliezer] do this [perform sorcery]? Did we not learn: If one performs a deed [of sorcery], he is liable? If it is only to teach, it is different. For it has been said: You shall not learn to perform the abominations of these nations (Deuteronomy 18:9). You shall not learn [that is,] in order to perform, but you may learn in order to understand and instruct [others].