Gideon Ofnir, The All-Knowing

When another creature activates an ability, you may pay {p/u}{u} to copy that ability. If you do select new targets, and put a Knowledge counter on Gideon.

{t}: Counter target instant or sorcery with mana value X or less unless its controller pays X, where X is the amount of Knowledge counters on Gideon.
{u}{u}{t}: Copy target instant or sorcery with mana value X and select new targets if any, where X is the amount of Knowledge counters on Gideon.