When does mana empty from the mana pool?

Mana empties from the mana pool at the end of each step and phase. This rule is crucial because it influences how players manage their resources during the game. When a step or phase ends, any unspent mana in the mana pool is lost, meaning players must use it strategically throughout their turns. This mechanic requires players to plan their actions accordingly to maximize their mana usage before it disappears.

In contrast, the other options are not accurate representations of when mana empties. Mana does not clear at the beginning of each player's turn, at the end of the game, or specifically tied to the casting of spells, but rather it is a consistent occurrence that happens at the end of every step and phase during all phases of gameplay. Understanding this rule is essential for effective gameplay and resource management in Magic: The Gathering.