Sananmuunnos ("Word transformation") 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. 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. Also, it is important to notice that most of these puns are somehow related with sexuality or other non-public matters. (e.g. excretion) Some purists actually define that a pair of words so transformed that isn't related to sexuality is not "official". (Also note that most of the examples in this article fall to this category of not appriopriate words.)
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.
Initial morae of two adjacent words are exchanged.
The "extra length" of a long vowel is a full mora, and thus stays in its original position, making the new vowel long.
If necessary, stilted diphthongs are converted to into allowed diphthongs as per phonotactics. The first vowel is the determinant for choosing the diphthong. The process preserves opening and closing diphthongs, e.g. the opening 'ie' is reflected as an opening 'uo'.
If necessary, vowel harmony is applied. As per vowel harmony, the initial syllable controls the kind of vowel selected.
That is, transformation is A, U, O into Ä, Y, Ö, if the former do not begin the word. Notice that information may be lost in this step, making it irreversible.
Exceptions are found, when the transformation would be irreversible.
It is possible (although not accepted by some "orthodox") to exchange only the initial consonants, if that is the only way to get a sensible result. Eg. palasokeri [p-ala/s-okeri] 'sugar lump' → salapokeri [s-ala/p-okeri] 'playing poker in secret' (*solapakeri would not mean anything).
Typically presented, spoonerisms are a kind of double entendre. Appropriately, the very term sananmuunnos is one; it becomes munansaannos, which can be understood as "small yield of penis".