Question: I'm stuck with these rulings in [Sunbird's Invocation](http://gatherer.wizards.com/Pages/Search/Default.aspx?name=%2b%5bSunbird%27s%5d%2b%5bInvocation%5d):

* For spells with {X} in their mana costs, use the value chosen for X
to determine the spell's converted mana cost.
* If a revealed card in your library has {X} in its mana cost, you must choose 0
as the value of X when casting it without paying its mana cost.

Does it mean spells with {X} sometimes have CMC 0 and sometimes X?

Additionnaly, can I play a land from the revealed cards?

Answer: 
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> Does it mean spells with {X} sometimes have CMC 0 and sometimes X?
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Yes, but thankfully it's simple:

* If a spell is on the stack, {X} is treated as the value the caster chose for it.
* Anywhere else, {X} is 0.

This comes from comprehensive rule 202:

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> **202.3d** When calculating the converted mana cost of an object with an {X} in its mana cost, X is treated as 0 while the object is not on the stack, and X is treated as the number chosen for it while the object is on the stack.
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However, the second ruling is referencing something else entirely: if you cast a card "without paying its mana cost", you always have to choose 0 for the value of {X}. You're not paying its mana, so you don't get to pick some other arbitrarily value. This means, more often than not, it is not worthwhile casting an {X} spell for free.

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> Additionnaly, can I play a land from the revealed cards?
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No. Sunbird's Invocation says you *cast* the card. Lands can't be cast, only played. See [Stromkirk Occultist](http://gatherer.wizards.com/Pages/Search/Default.aspx?name=%2b%5bStromkirk%20Occultist%5d) for an example of a card that lets you play lands.
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> **Does it mean spells with {X} sometimes have CMC 0 and sometimes X?**
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No, an X for a *spell* is whatever was chosen. An X for a *card* is 0.

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> 111.1. A spell is a card on the stack. 
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For something to be a spell, it has to be put on the stack, and to put it on the stack, you have to pay its cost. Whenever you put a spell with X in its cost on the stack, you have to choose what X is. Whatever you choose for X, that's what's used for Sunbird's Invocation.

When you reveal cards, those aren't spells yet, because they haven't been put on the stack. Since no X has been chosen, X is 0.

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> **[Additionally], can I play a land from the revealed cards?**
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No, it says that you can "cast" a card, and lands aren't cast.

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> 305.1. A player who has priority may play a land card from their hand during a main phase of their turn when the stack is empty. Playing a land is a special action; it doesn't use the stack (see rule 115). Rather, the player simply puts the land onto the battlefield. Since the land doesn't go on the stack, it is never a spell, and players can't respond to it with instants or activated abilities. 
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