Planeswalker's Mischief

[3][U]: Target opponent reveals a card at random from his/her hand. If it's an instant/sorcery card, exile it. You may cast it without paying its mana cost for as long as it remains exiled. (If it has X in its mana cost, X is 0.) At the beginning of the next end step, if you haven't cast it, return it to its owner's hand. Activate this ability only any time you could cast a sorcery.