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Brain fart question... does your mana pool only empty at the end of turn or does it empty at each step (e.g. after upkeep ends, etc.).
I looked it up but was a little confused... (probably because it's 2:33 am and i should be in bed),
Mana pools empty as steps and phases end.
The 5 phases are Beginning Phase (steps: untap, upkeep, draw), Precombat Main Phase, Combat Phase (steps: beginning of combat, declare attackers, declare blockers, combat damage, end of combat), Postcombat Main Phase, Ending Phase (steps: end, cleanup).
You can tap those two untapped lands at any time to produce mana and the use it to cast spells and activate abilities. What you can't do is tap them in one step or phase for use in a later step or phase.
As an example, you can't tap those two lands during your opponent's upkeep for mana and then cast a spell using that mana during their precombat main phase. Your mana pool will have emptied at the end of the upkeep step and that mana you added at that time will no longer be in your mana pool.
I understand. Is there any benefit to tapping mana in the beginning of a phase, rather than actually tapping it when it is needed? It seems like this rule came about because people where tapping mana and then using it in later phases - but why would they do that?
One scenario I can think of is to protect against the possibility of having one of their lands destroyed by a spell (since they would already have it tapped i.e. the mana is at their disposal).
With basic lands there's usually no need to tap them ahead of time, it usually comes into play with non basic lands and other effects that give you mana.
Azorius Chancery gives you 2 mana, so if you only needed the blue and not the white for instance, you'd lose it.
Braid of Fire gives you mana during your upkeep, so you have to use it then or lose it.
Tapping in response to a land being removed is common, if you have something to spend it on.
Emptying the mana pool at the end of each step and phase was originally added to the game to avoid dealing with certain memory issues that the playtesters experienced. Before the rule was added to the playtest version of the game, they had situations where a player would add a bunch of mana to his pool at the beginning of the turn, and then later in the turn the players would have trouble remembering how much he had used and what he had left. To discourage players from getting themselves in this situation, the regular emptying of the mana pool was added to the rules, as well as mana burn (which was taken out of the game in the M10 Rules Update).