I don’t think it was in the book, either…
Frodo is pondering the ring, and the heavy responsibility it represents. He pours some milk on the table, and stares into it morosely.
He begins to have visions.
Gandalf comes into the room, and orders the young hobbit to desist immediately, that such a technique can only lead to false or meaningless visions.
“Why” asks the bewildered hobbit.
“It’s no use scrying over spilled milk.”
Yes, that I know that was dairy bad of me, it’s worse if you know Scrying is a Wiccan/New Age technique of meditation by unfocusing the mind and staring into a mirror, water, fire, or what have you, to see what visions or thougts come.