Who says rhyme has to have reason?

Somewhere it lurks beneath the surface of “Runny Babbit: A Billy Sook,” but in Shel Silverstein’s posthumous volume of kid’s poetry, logic is far from the only raison d’etre.

What better way to mark National Poetry Month than with sensory celebration?

Not only written but also illustrated by Silverstein, this delightfully dizzy book is filled with spoonerisms, a type of malapropism that results in the transposition of parts of words — hence, a simple bunny rabbit becomes Runny Babbit. And fortunately for Runny, Silverstein is able to make this language into a fulfilling way of life for the hapless rabbit.

The gifted writer has a knack for creating a veritable barrage of sounds that offer a strange sort of sense even if they are not “translated” into their true forms. For example, Runny goes out to find some “taisin roast,” only to be faced with “sea poup.” The mixed-up words actually sound real, even expressive, obviously chosen for their immediate effect as well as the correct transposed meaning.

Either way, they provoke an acute remotional eaction. (Correct translation: laughter.)

Silverstein, perhaps best known for his beloved parable “The Giving Tree,” is such a nimble thinker that readers young and old have to run to keep up. He delights in creating situations fraught with possibilities, such as the event “Runny Bakes a Tath.”

A wild situation inevitably gets wilder until at last it climaxes with Runny “shrinking the dampoo.” The most amazing thing of all is that the newly-coined phrase makes sense as it stands.

The news release for “Runny Babbit” (HarperCollins, $17.99) notes that Silverstein was, among many things, a Grammy-winning and Oscar-nominated songwriter — information that isn’t surprising, given the musicality of his language. He comes up with the melodic phrase “Calley At and Kittle Litten” and creates a “Rig Bomance” poem that revels in sweet phrases like “Boney-Hun” and “Dovey Lear.”

The accompanying art is at once comic and unabashedly sentimental.

A critic’s advice: Hurry to get this tour-de-force of bunny babble. Children of all ages immediately will begin to read or listen gleefully, an all-too-rare occurrence. Here is a hero who thinks like a kid, acts like a kid, and talks the way all kids really want to talk.

Indulging children in inspired nonsense is the best reason for these rhymes. Buying the book is something that, like the book itself, ultimately makes perfect sense.