This article is from the Magic: The Gathering Rules FAQ, by Patrik Linell (pls@claymore.nu) with numerous contributions by others.
Because a spell or ability cannot resolve as soon as it is cast,
there is a time when abilities and instant spells can be cast "in response"
to it. In other words, they are announced and put onto the stack above it.
Things done in response to a spell or ability will resolve first. The
spells most commonly cast in response to something are those that will
counter it, for example a Counterspell.
 
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