Rule 700.14

Some abilities trigger “Whenever you expend N.” A player expends N if they pay a cost to cast a spell and the amount of mana that player spent this turn to cast spells prior to paying that cost was less than N and became at least N after paying that cost. Example: A player casts Bark-Knuckle Boxer, which costs {1}{G} and reads “Whenever you expend 4, Bark-Knuckle Boxer gains indestructible until end of turn.” After it resolves, that play casts Divination, a spell that costs {2}{U}. Prior to paying the cost to cast Divination, that player has spent two mana to cast spells this turn. After paying the cost, they have spent five mana to cast spells this turn. Since they have now spent at least four mana to cast spells this turn, Bark-Knuckle Boxer’s ability triggers.

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