Isochron Scepter

You cast the copy while the ability is resolving and still on the stack. You can't wait to cast it later in the turn.

If you don't want to cast the copy, you can choose not to; the copy ceases to exist the next time state-based actions are checked.

If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.