If all players pass in succession (that is, if all players pass without taking any actions in between passing), the spell or ability on top of the stack resolves or, if the stack is empty, the phase or step ends.
July 16, 2010 · Magic 2011
Some costs include an {X} or an X. See rule 107.3.
February 1, 2010 · Worldwake
If a cost or effect allows a player to pay an amount of life greater than 0, the player may do so only if his or her life total is greater than or equal to the amount of the payment. If a player pays life, the payment is subtracted from his or her life total; in other words, the player loses that much life. (Players can always pay 0 life.)