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: 
> 
> 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:

> 
> **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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> 
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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.

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

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.
Yes, CMC only includes X when the spell is on the stack:

> 
> 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.
> 
> 
> 

This explains your first quoted ruling - the CMC of the spell triggering your Sunbird's Invocation includes X because it's on the stack.

The second one is actually subtly different. It's not just about the CMC of the revealed card (which does treat X as 0) but also about what happens when you cast the spell without paying its mana cost:

> 
> 107.3b If a player is casting a spell that has an {X} in its mana cost, the value of X isn’t defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0.
> 
> 
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So for example, if you cast a [Sanguine Sacrament](http://gatherer.wizards.com/Pages/Search/Default.aspx?name=%2b%5bSanguine%20Sacrament%5d) (mana cost {X}{W}{W}) with X=5, its CMC is 7, so Sunbird's Invocation will reveal 7 cards and let you cast something with CMC up to 7.

However, if you find another [Sanguine Sacrament](http://gatherer.wizards.com/Pages/Search/Default.aspx?name=%2b%5bSanguine%20Sacrament%5d) among those revealed cards, you would only be able to cast it with X=0, even though you're allowed to cast spells with higher CMC.

---

You can't cast lands, so you can't play lands from the revealed cards:

> 
> You may **cast** a card revealed this way...
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> 

If a card lets you play lands, it needs to say that you can "play" the card, not "cast" it.