Rule 602.2

To activate an ability is to put it onto the stack and pay its costs, so that it will eventually resolve and have its effect. Only an object's controller (or its owner, if it doesn't have a controller) can activate its activated ability unless the object specifically says otherwise. Activating an ability follows the steps listed below, in order. If, at any point during the activation of an ability, a player is unable to comply with any of those steps, the ability was activated illegally; the game returns to the moment before that ability started to be activated (see rule 714, "Handling Illegal Actions"). Announcements and payments can't be altered after they've been made.

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