Rashi on Deuteronomy 18:9/רש"י על דברים י״ח:ט׳

1לא תלמד לעשות THOU SHALT NOT LEARN TO DO [AFTER THE ABOMINATIONS OF THOSE NATIONS] — thou shalt not learn to do, but you may learn their practices in order to understand them and to teach others, that is to say, to understand their doings, how depraved they are, and thus to be able to teach thy children, “Do not so and so because these are the religious observances of the heathens!” (Sifrei Devarim 170:3; Sanhedrin 68a.)