Who can look at face-down creatures?
You can look at any face-down creatures you control. You can’t look at any your opponents control. Also, whenever a face-down creature leaves play, it’s revealed to everyone. At the end of the game, any face-down creatures in play are revealed to everyone. This prevents people cheating by playing cards without morph face down. Once you have a face-down creature in play, you have the option to turn it face up. You can do this any time you have priority. Just pay the morph cost, and turn it face up. This isn’t an activated ability, and doesn’t use the stack, so your opponent can’t respond to it or counter it. Why can’t my opponent respond to me turning a creature face up? Partly because it would suck if you paid to turn your creature face up, and your opponent responded with a Shock to kill the 2/2 face-down creature. Partly because it would be difficult to confirm that you had paid the correct amount of mana without turning the card face up to prove it. Your 6/6 creature will always sur