Mana can come from several sources, but sites are the most common. You use this mana to cast spells or activate abilities.
When you want to spend mana, you total the mana from all sources you currently control and subtract how much you’ve already spent this turn. For example, if you currently have 5 mana from sources you control and have spent 3, you can spend 2 more.
You only have mana on your own turn and cannot spend mana on your opponent's turn.
Sometimes you gain or lose control of a mana source during your turn, which applies immediately. For example, I have 3 sites including . I sacrifice my Sinkhole, then play another site, bringing my total sites back up to 3. I can cast something that costs 3, not 4, since I have a total of 3 mana from all sources I currently control.
As another example, I have 3 sites including Sinkhole. I cast something that costs 3, then sacrifice the Sinkhole. That’s fine! The rule is simply that at the moment you want to spend mana, you check to see if you have enough. In this example, if I then played another site, bringing my total back to 3, I’d still have 0 available mana, since the game notices that I have 3 sites right now and already cast something that cost 3.
As another example, I have 4 sites. I play (3 mana), gaining control of my opponent’s . I can now cast something that costs 2, since I have 5 mana from my sources (4 sites + 1 core) and I’ve spent 3 mana so far.
Any mana source that “provides” mana, including sites, must be in the realm and controlled by you to count toward your total available mana, e.g. .
Other effects may allow you to “gain” mana, which function as independent mana sources. For example, if you sacrifice , you have 1 extra mana when totaling your available mana that turn, even though Ferryman’s Coin isn’t in the realm after being sacrificed. This applies to all effects that cause you to “gain” mana, such as tapping (even if they leave the realm after tapping), resolving an (which doesn’t go into the realm at all because it’s a magic), or the 1 bonus mana from ’s genesis effect (even if the site itself leaves the realm that turn).