Ghastly Reflection

Target opponent reveals their hand. You may
exile a card from their hand or their graveyard.
You may look at and cast that card for as long as
it remains exiled, and you may spend mana as
though it were mana of any type to cast that spell.
He too saw the image in the water; but he looked up at
once, and became aware of the lovely Lassie who sat
there up in the tree.