Exhaust abilities can be activated any time you could normally activate an ability.

If an exhaust ability of a permanent is activated, and then that permanent leaves the battlefield and returns to the battlefield, it becomes a new object so its exhaust ability can be activated again.

If an ability triggers whenever you activate an exhaust ability, that ability resolves before the exhaust ability resolves.

{E} is the energy symbol. It represents one energy counter.

Energy counters are a kind of counter that a player may have. They’re not associated with any specific permanents.

Keep track of how many energy counters each player has. Potential ways to track this include writing them down on paper or using dice, but any method that is clear and mutually agreeable is fine.

If an effect says you get one or more {E}, you get that many energy counters. To pay one or more {E}, you lose that many energy counters. You can’t pay more energy counters than you have. Any effects that interact with counters a player gets, has, or loses can interact with energy counters.

Energy counters aren’t mana. They don’t go away as steps, phases, and turns end, and effects that add mana “of any type” can’t give you energy counters.

Some spells and abilities say that you “may pay” a certain amount of {E}. You can’t pay that amount multiple times to multiply the effect. You simply choose whether or not to pay that amount of {E} as the ability resolves.

Some spells and abilities say that you “may pay” a certain amount of {E}, then describes an effect that happens “if you do.” In that case, no player may take actions to try to stop the ability’s effect after you make your choice. If the payment is instead followed by the phrase “when you do” followed by an effect, then after you choose to pay {E}, a new triggered ability called a reflexive trigger is put on the stack, and you choose targets for the reflexive trigger (if the effect asks for targets). Players may respond as normal to this new triggered ability. Once that reflexive trigger resolves, the effect will finally occur.

Some spells and abilities which give you {E} or say you “may pay” some amount of {E} require one or more targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won’t resolve. None of its effects will happen, you won’t get any {E} from it, and you won’t get the chance to pay any {E} for its effects even if you want to.