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When Thriving Turtle enters the battlefield, you get {E}{E} (two energy counters).
Whenever Thriving Turtle attacks, you may pay {E}{E}. If you do, put a +1/+1 counter on it.
Energy counters are a kind of counter that a player may have. They're not associated with specific permanents. (Other kinds of counters that players may have include poison and experience.)
Energy counters aren't mana. They don't go away as steps, phases, and turns end, and effects that add mana “of any type” to your mana pool can't give you energy counters.
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. Any effects that interact with counters a player gets, has, or loses can interact with energy counters.
Keep careful track of how many energy counters each player has. You may do so by keeping a running count on paper, by using a die, or by any other clear and mutually agreeable method.
Some triggered abilities state 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, and no player may take actions to try to stop the ability's effect after you make your choice.
You can't pay more energy counters than you have.
{E} is the energy symbol. It represents one energy counter.
