text: Although turning a face-down creature with morph face up doesn't use the stack, Rattleclaw Mystic's triggered ability does use the stack, and players can respond to that ability. Notably, you can't turn Rattleclaw Mystic face up while casting a spell or activating an ability. You'll need to turn Rattleclaw Mystic face up and have its triggered ability resolve before you can spend that mana.