Even as he spoke, they saw, as if she came in answer to their words, the Lady Galadriel approaching. Tall and white and fair she walked beneath the trees. She spoke no word, but beckoned to them.
Turning aside, she led them toward the southern slopes of the hill of Caras Galadon, and passing through a high green hedge they came into an enclosed garden. No trees grew there, and it lay open to the sky. The evening star had risen and was shinning with white fire above the western woods. Down a long flight of steps the Lady went into a deep green hollow, through which ran murmuring the silver stream that issued from the 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 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.’
The air was very still, and the dell was dark, and the Elf-lady beside him was tall and pale. 'What shall we look for, and what shall we see?' asked Frodo, filled with awe.
'Many things I can command the Mirror to reveal,' she answered, 'and to some I can show what they desire to see. But the Mirror will also show things unbidden, and those are often stranger and more profitable than things which we wish to behold. What you will see, if you leave the Mirror free to work, I cannot tell. For it shows things that were, and things that are, and things that yet may be. But which it is that he sees, even the wisest cannot always tell. Do you wish to look?'
Quoted from J.R.R. Tolkiens The Fellowship of the Ring pages 467-468.