Rule 113.3c

Triggered abilities have a trigger condition and an effect. They are written as “[Trigger condition], [effect],” and include (and usually begin with) the word “when,” “whenever,” or “at.” Whenever the trigger event occurs, the ability is put on the stack the next time a player would receive priority and stays there until it’s countered, it resolves, or it otherwise leaves the stack. See rule 603, “Handling Triggered Abilities.”