Wikipedia
Sananmuunnos ("Word transformation"), sometimes kääntösana, is a sort of verbal play in the Finnish language, similar to spoonerisms in English.
Special to Finnish is a narrow phoneme inventory and vowel harmony. As Finnish is a mora-divided language, it is morae that are exchanged, not syllables (often a mora is also a syllable in a Finnish word, but not always). Also, Finnish inflectional and derivational morphology is extensive, thus applying a suffix from another word often produces a valid word. This leads to large number of possible spoonerisms. Much of practical sananmuunnos wordplay revolves around obscene double entendre expressed by spoonerism.
Several books have been written. Some have whole stories with multiple puns in one sentence, for example. Most have a "vocabulary" in the back, usually a hundred or more word pairs long.