Mana ability

A mana ability is an activated ability that creates mana (such as with Llanowar Elves, Birds of Paradise, and Ur-Golem's Eye), or a triggered ability that triggers off of mana creation and makes more mana itself (such as with Overgrowth or the last ability on Gauntlet of Power).

Mana abilities are among the few activated or triggered abilities that don't use the stack or require passing priority to resolve, so they cannot be responded to or targeted. As soon as the ability goes off, the mana is created. In practice, this simply means that a player can use mana abilities any time they please, usually while playing a spell or ability and paying the cost. Other effects of the mana ability, such as the damage that sometimes happens when using Adarkar Wastes, for example, also occur immediately and can't be responded to.

Spells that generate mana, such as Seething Song, use the stack normally, and can be responded to and targeted.

Rules
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Rulings

 * If a mana ability can't produce any mana, it's still legal to play the ability, but no mana will be produced on resolution.
 * Playing a mana ability that happens to produce no mana still counts as playing that ability. Some abilities trigger when a land is tapped for mana, and they will trigger when a land is tapped for a mana ability even if that ability generates zero mana.