[Outdated; Inaccurate] JJ's Rules Lesson -- The Stack

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Originally by Jeff Jordan:

RULES LESSON - THE STACK
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One of the most powerful - and misunderstood - features of Magic is
the ability to respond to an action taken by your opponent, and have
the response occur first. Not all actions can be responded to like
this. The only things that can are items placed on the stack. In fact,
the phrases "I respond to that with this" and "before you do that, I
do this" literally mean "I put this on the stack on top of that."
The actions that use the stack are spells (the card is physically put
in the stack zone), triggered or activated abilities (an imaginary card
called a pseudospell in put in the stack zone), and the assignment of
combat damage. When both players decline to respond to something on the
stack, that "something" resolves, and both players get a chance to
respond to whatever was under it on the stack.

The stack works on a system of priority. First one player can put as
many things on the stack as he can announce, and finally he passes
priority to his opponent. She can then put as many things on as she
can announce before passing priority back. This repeats until both
players pass, the top item on the stack resolves, and the main player
gets priority again.

One action that seems like it uses the stack, but technically does not,
is going to the next step or phase. If both players pass while the
stack is empty, the current phase or step ends. Players will often say
"I declare my attack phase" when they really mean "I pass priority to
you with the stack empty."

All the things that you do while announcing the spell or ability do not
go on the stack, and cannot be responded to. This includes choosing
targets, getting mana, and paying all costs. Costs include tapping,
sacrificing, discarding, or whatever else is called for by the spell or
ability. You cannot do anything to stop one of these actions by an
opponent. This can be summarized in what I call "the STACK rule"

Only five things can be responded to in Magic:
S: a Spell on the stack
T: a Triggered ability on the stack
A: an Activated ability on the stack
C: the assignment of Combat damage
K: saying you are ready to sKip to the next step or phase

(If anybody can phrase a better "K" for this, I'm open to suggestions.)