Rule 605.4a

A triggered mana ability doesn’t go on the stack, so it can’t be targeted, countered, or otherwise responded to. Rather, it resolves immediately after the mana ability that triggered it, without waiting for priority. Example: An enchantment reads, “Whenever a player taps a land for mana, that player adds one mana of any type that land produced.” If a player taps lands for mana while casting a spell, the additional mana is added immediately and can be used to pay for the spell.

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