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609.3. If an effect attempts to do something impossible, it does only as much as possible.

Because of this rule, I would expect Frost Breath to not work on creatures that are tapped. Why is it possible to tap a tapped creature, but it’s impossible to untap an untapped land with e.g. Teferi, Hero of Dominaria? This seems to be a contradiction to me. Can someone explain why it’s not? Thanks!

  • Re "Why is it possible to tap a tapped creature", It's not possible. That's why basic lands can't produce mana when they're already tapped. And if there were an ability that said "tap or untap <something>", you'd be force to toggle <something>'s tap status. What makes you think it's possible to tap something that's already tapped? – ikegami Jun 19 '18 at 0:45
  • Maybe you're wondering why FB can target tapped creatures. It's because the targeting instruction is "target creatures", not "target untapped creatures". – ikegami Jun 19 '18 at 1:12
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The main difference between those cards is that Frost Breath targets the permanents and Teferi's ability does not.

Frost Breath's text says

Tap up to two target creatures. Those creatures don't untap during their controller's next untap step.

The targets are chosen when you cast the spell, and since it just says "target creature", you can choose any creature, even a tapped one. Once the spell resolves, it tries to tap those specific creatures. If it can't, it just moves on to the next instruction.

Teferi's ability, on the other hand, does not target. It just says

Draw a card. At the beginning of the next end step, untap two lands.

This effect just tries to untap any two lands, so you have to choose lands that can actually be untapped. This effect still follows the quoted rule 609.3; if there is only one tapped land on the battlefield when the ability resolves, you untap just that land.

You have to choose tapped lands because rule 701.20b says

To untap a permanent, rotate it back to the upright position from a sideways position. Only tapped permanents can be untapped.

and in part of the description of the process of resolving a spell or ability, rule 608.2d says

If an effect of a spell or ability offers any choices other than choices already made as part of casting the spell, activating the ability, or otherwise putting the spell or ability on the stack, the player announces these while applying the effect. The player can’t choose an option that’s illegal or impossible [...]

It is impossible to untap a permanent that is already untapped, so you can't choose to do so.

  • Maybe a new question, but what rule says how Teferi works here? It's not clear from just the wording that "untap two lands" requires you to choose two tapped lands. – GendoIkari Jun 18 '18 at 21:02
  • I added a rule quotation. Does that answer your question? – murgatroid99 Jun 18 '18 at 21:05
  • Is there a rules that explains the process of making a choice in a case like this? How do we know that "untap two lands" requires you to choose two lands that can be untapped? The rules are very specific about the process of choosing targets, for example. – GendoIkari Jun 18 '18 at 21:08
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    Some of the same issues come up with Assault Suit. If you control a creature with Assault Suit, and an effect says "each player sacrifices a creature", you can't choose that creature and then say "oh well, I can't sacrifice it, so I guess I sacrifice nothing". If a game instructs you to take an action on an object of your choice, you have to choose an object on which you are actually allowed to take that action. – murgatroid99 Jun 18 '18 at 21:34
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    It's specifically not "choose two lands and untap them" (which is how it might be worded to allow untapped lands); it's "carry out the action of untapping two lands". – Samthere Jun 19 '18 at 10:03

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