The greatest children's books, it seems, are the ones children still read when they are grown-ups. This was the brilliance of Dr. Seuss and A. A. Milne. And with no exception, it was the brilliance of Shel Silverstein.

Few realize that Silverstein, who passed away last Monday, spent many years writing and illustrating for adults. In the 1950s, he served overseas in the military, drawing cartoons for Stars and Stripes. After returning to the United States, he began drawing cartoons for Playboy. In fact, he once told a close friend he never intended to become a children's author. Even after Silverstein published his first children's work in 1963, he continued to write poems and song lyrics for adults.

Ironically, this is precisely why Silverstein will be remembered most fondly for his children's poetry. He was a master of the form precisely because his verse was both innocently charming and irreverently naughty. Adults and children alike could take delight in the twisted sense of humor found in Silverstein's poems and illustrations.

Who could forget such wonderful characters like Dentist Dan, who "filks my cavakies/Wid choclut cangy...The graygest nentis in the lan." Or poor Peggy Ann McKay (sick with the measles, mumps, a gash, a rash and purple bumps), Backwards Bill (who puts on his underwear over his clothes) and Sarah Cynthia Sylvia Stout (who never had a playmate because she never took the garbage out).

Only Silverstein could get away with tales of anteaters who turned out to be an aunteaters ("And now my uncle's mad") and the boy who, having nothing to put into his stew, climbs into the pot and makes a stew of himself ("I'll sing while I simmer, I'll smile while I'm stewing/I'll taste myself often to see how I'm doing.")

Perhaps Silverstein's greatest work was The Giving Tree, which told a deceptively simple story about a tree "who loved a little boy." The tree, in her love for the boy, gives everything of herself--quite literally--to a boy who selfishly takes.

For children, The Giving Tree is a metaphor for unrequited parental love. For adults, it has been interpreted as a everything from a religious parable to a cynical look at adulthood. A symposium on the book was held in 1995, where distinguished scholars debated the book's underlying meaning.

The Giving Tree is just one example of how Silverstein's work for children was embraced by children and adults alike. And this is what made him a master. In 1975, Silverstein told Publisher's Weekly, "I would hope that people, no matter what age, would find something to identify with in my books, pick up one and experience a personal sense of discovery."

And who knows? That discovery might turn out to be a bit twisted.