Why Do They Do That? : One child stands on his head, another eats like a dog. Don’t worry--they’re fine.

Mitchell can’t stop washing his hands. Philip has trouble keeping his clothes on--even in cold weather. And Luke goes everywhere with a pair of men’s bikini underpants around his neck.

Bizarre behavior? Maybe by adult standards, but not for little kids. For 4-year-old Philip, 2-year-old Luke and 3-year-old Mitchell, it’s just a part of growing up.

Parents may be worried when children’s habits mimic frightening adult disorders, but the experts say: Relax. If your child’s actions are not endangering him, her or anybody else, they’re probably OK.

“Each family is unique in the amount of differentness they’re willing and able to tolerate,” says Alan Entin, past president of the American Psychological Assn.’s division of family psychology.

“Unless a child’s behavior is not safe,” says Entin, “or consistently gets the child in trouble with teachers or peers, individuality can really be quite wonderful.”

When grown-ups wash their hands 30 or 40 times a day and they’re not doctors or auto mechanics, they’re considered obsessive-compulsive. When Mitchell does it in one of his family’s four bathrooms at home on Long Island, he’s 1) playing with water; 2) practicing a newly acquired life skill and 3) making a very large mess in the bathroom. All of which are perfectly normal for a healthy 3-year-old.

Taking off one’s clothes, all one’s clothes, at the drop of a hat, shoe or snowflake is also “age-appropriate behavior,” say the experts.

But that doesn’t make it any easier for Philip’s parents, who have blushed more than once to have their Altadena neighbors see their little boy--all of their little boy--out climbing on the swing set.

Luke took to wearing his father’s underpants the first time his scientist dad left home for a business trip.

“He really missed him,” recalls Luke’s mother, “but found great comfort by getting out those pants and hanging them--loosely, thank heavens--around his neck.”

This new accessory grew so comfortable that even after the father’s return, Luke continued to wear a pair of his underpants. For a time, he wore them everywhere, including nursery school. Now, that he’s older--2 1/2--he leaves them in his cubby while in school and puts them on again when it’s time to go home.

Luke’s underpants--like Mitchell’s hand-washing and Philip’s nakedness--have never interfered with his socialization or success in school. In fact, the other children don’t seem to notice.

Children, says veteran teacher and author Vivian Gussin Paley, “find eccentricities understandable and not at all strange.”

In her new book, “The Boy Who Would Be a Helicopter,” Paley writes about a group of preschoolers who accommodate and genuinely accept a boy who, for much of the school year, cannot separate himself from his toy helicopter.

The book, which explores how Paley uses storytelling in her Chicago classroom to encourage preschoolers’ fantasy play, centers on the true story of Jason, who is so involved in his helicopter play that at first it isolates him from the other children.

Jason repeats and repeats his helicopter monologue:

“This blade is turning around now you’re going faster now you’re going faster now you’re going to crash now you’re going off the ground now you’re going up up up now you’re going loud br-r-r-roooom now you’re going to land pshshsh OK all safely.”

“I want to I always have to be a helicopter,” Jason tells Paley, who slowly helps Jason join the other children in play. And when at last Jason allows another child into his fantasy, the boys fly together in formation, chins forward, arms spinning.

According to Paley, when it comes to children, the more dramatic the behavior, the better. “What is wrong with twirling and fixing your blades or falling down if this is part of your act? You are supposed to be as ostentatiously dramatic as you can be at all times. So say the children by their own actions.”

When Eric Bulgarelli of Tujunga was about 4, he enjoyed making dramatic entrances to the playground in one of two favorite outfits: a lacy white bridal gown with train and matching gloves or a black cowboy suit with black hat, black boots and, yes, matching black gloves.

His appearance in such clothes, as well as the aplomb with which he wore them, helped make him the most exciting and charismatic boy in his preschool.

His parents and his teachers enjoyed the drama of Eric’s wardrobes but after some months found it was becoming disruptive. It was then that both teachers and parents decided to put more variety into his clothing choices. But he was still permitted to wear black on certain “dress-up” days and allowed to don the wedding gown at will.

The idiosyncratic nature of children’s preferences makes childhood a non-threatening time to try out different behaviors, to act out different fantasies, say child psychologists.

According to Entin, who practices in Richmond, Va., when there are problems with children’s behavior, the problem may often rest with the parent, not the child. “Most of what we’re talking about (among preschoolers) is quite benign. I mean, does it really matter if little boys that age dress up in pink tutus or wear their mothers’ make-up? No, of course it doesn’t.”

Entin’s wife, Phyllis, has taught preschool for almost 20 years and believes some problems arise more from parents’ expectations than from children’s behavior. But there are solutions. “One family let their little girl dress herself, and her choices were, by adult standards, quite strange, so they pinned a note to her dress. It was a disclaimer saying her dress did not represent the opinion of the management.”

“Eating, sleeping, toilet training, talking, these are the issues the child has total control over,” said Phyllis Entin. “There is no point for parents to try to take control in those areas, because the child will always win.”

When Wendy McCarren was 4, her mother decided it was time for her to go to preschool. But every morning when she dropped her off, Wendy threw up. After a week, Wendy won and quit preschool. A year later, she had no problems beginning kindergarten.

Timmy James went through a phase around the age of 3, when he wanted to be treated like a dog. Literally. He insisted on being fed milk and crunchy cereal from saucers on the floor.

His mother obliged, and the interest in living a dog’s life eventually faded.

Philip and Toni Brooks used the same non-judgmental response to son Danny’s habit of standing on his head whenever he felt stressed.

Both parents are therapists in San Francisco and were surprised but not worried when 5-year-old Danny, already an accomplished gymnast, began to stand on his head for one, two, three minutes at a time.

“The first time he did it on a moving train, we were, well, rather impressed that he could hold his balance that well,” said Danny’s mother. “As it turned out, he meant to impress a little girl up the aisle. It was easier to stand on his head than to figure out how to start a conversation with her.”

Attention-getting may often be the motivation for eccentric behavior, says author Paley. And that’s not bad.

Jason, for example, used his helicopter persona to “tell his story,” says Paley. “Why else does he roar his motor and openly complain about broken blades if not to capture our attention?”