How does a spoonerism change meaning by switching sounds between words?

A spoonerism is a rhetorical device or verbal slip in which the initial sounds, letters, or syllables of two or more words are transposed.123

By switching these phonetic elements, the original meaning of a phrase is replaced by a new, often humorous or nonsensical combination of words. Because the resulting words are typically familiar, the shift creates a jarring or comic effect that contrasts with the speaker's intended message.12

For example:
* "Half-formed wish" becomes "half-warmed fish".13
* "Crushing blow" becomes "blushing crow".12
* "Lighting a fire" becomes "fighting a liar".2

While often occurring as accidental speech errors, these transpositions are sometimes employed intentionally to produce a specific comic effect. The term is named after William Archibald Spooner, an educator who was famously associated with these unintentional verbal blunders.123