Mirror of Galadriel

"Down a long flight of steps the Lady went into a deep green hollow, through which ran murmuring the silver stream that issued from a fountain on the hill. At the bottom, upon a low pedestal carved like a branching tree, stood a basin of silver, wide and shallow, and beside it stood a silver ewer. . . With water from the stream Galadriel filled the basin to the brim and breathed on it, and when the water was still again, she spoke. 'Here is the Mirror of Galadriel,' she said. 'I have brought you here so that you may look in it, if you will." --LotR, Mirror of Galadriel

This mirror was what hobbits would call 'elf-magic'. In it one could see many things, but could not control what he saw, nor tell whether what he saw was in the past, present, or if it had not yet happened. In this mirror Sam saw trees being chopped down in the Shire, and Frodo saw Mordor and the Eye of Sauron.